The Separation of Powers is a principle of the constitution
three branches; the executive, the legislature and the judiciary, should be SEPARATE, UNIQUE and EQUAL. There should be a clear separation between the people and functions of the legislature, executive and judiciary, otherwise Montesquieu said there will be “tyranny".
However this does not mean that the bodies should have no power over each other, Blackstone argued that what is required is a “check and balance" system between them, this is referred to as the theory of mixed government. If the branches were completely separate it would be unworkable, particularly as the Parliament is Supreme. There should be sufficient interplay between the branches, for example, the executive proposes legislation, Parliament debates and passes the law, and the judiciary uphold the Acts of Parliament.
The members of the government must be drawn entirely from the Houses of Parliament. To put it another way, the executive is completely made up of people who are already members of the legislature. Government Ministers continue to sit as “normal" members of the legislature in addition to their ministerial responsibilities. This is a very clear example of there being no formal separation of powers. By convention the PM must be a member of the House of Commons.
The British electoral system combined with the Party system produces a dominant executive that actually sits within the legislature.
The legislature has delegated powers to Ministers to create statutory instruments (delegated legislation). Therefore individual members of the executive can themselves actually legislate. However this is subject to Parliamentary scrutiny.
In ex parte Fire Brigade  the court held that it was unlawful for the Home Secretary to introduce changes to a scheme which were incompatible with an Act of Parliament.
Many of the Queen’s prerogative powers are now actually used by her Ministers. Ministers can use their prerogative powers to legislate without the consent of Parliament. However to some extent this is limited by judicial review.
Despite the lack of separation of people, many people who are already members of the “executive" such as civil servants, the police and members of the armed forces (as well as members of the judiciary) are barred from becoming an MP and joining the legislature by the House of Commons Disqualification Act . This Act also limits the number of MPs that can become ministers as a check on executive power.
CHECKS ON EXECUTIVE BY LEGISLATURE
To hold the executive to account there are several “checks" on executive power.
Every government is dependent upon parliament for its survival in office – no matter how big the majority the government has, if they lose a vote of confidence convention forces the government to resign and a general election to be called as in the Callaghan Government of 1979.
Parliamentary procedures are designed to scrutinise legislative proposals and the government will not always get its own way, PACE 1984 was substantially altered due to pressure from MPs from all sides.
Question Time, debates and select committees all ensure the accountability of government to Parliament.
The opposition gets several days per Parliamentary session to hold debates on subjects of their choosing.
The House of Lords can amend and delay most Bills for up to a year before the Parliament Acts 1911 & 1949 take effect (bypassing the Lords and going straight to the Queen for the Royal Assent). Rather than have its proposals delayed the government may prefer to compromise its proposals or accept amendments made by the House of Lords. This shows the legislature holding the executive to account.
Most prerogative powers are exercised by the government in the name of the crown. In CCSU v Minister of state for Civil Service , (the GCHQ case) the House of Lords ruled that executive power can be judicially reviewed even if it comes from a royal prerogative, but there are many subjects which judges should not review as it is for the democratically elected executive to decide. For example judges will not review the decision to go to war.
There is a convention that members of the executive should not criticise judges. This is often ignored – Mrs Thatcher criticised the light sentence given to a child molester. Also this rule only applies to members of the executive, not normal MPs. Also a judge who said a rape victim was guilty of contributory negligence was criticised in Parliament.
Judges are not chosen on party political grounds.
The pay of judges is set independently – to preserve judicial independence.
Judicial Review is designed to keep those people or bodies that have had powers delegated to them within those powers. So if a minister or a local authority exceeds the powers that Parliament has given it, the courts will nullify the decision and require that the decision maker makes a decision according to the correct procedure. As judicial review is concerned with the process of taking the decision rather than the merits of the decision itself, it could be said that the judiciary are upholding the will of Parliament in controlling the powers it has delegated.
Judicial review is paradoxical, because it could be said that the judiciary are upholding both the Rule of Law and the Supremacy of Parliament, but at the same time they are acting as a check on executive power, arguably infringing the Separation of Powers! For example if Parliament gives powers to a Minister to “act as he sees fit" to what extent is it proper for a court to question his decision-making power?
In R v SSHD ex parte Anderson  it was held that the Home Secretary exercising judicial functions in fixing the sentence of a murderer was a breach of Article 6 ECHR.
Judges are often appointed by the executive to chair official enquiries, such as into the death of David Kelly. Judicial enquiries leave judges open to criticism.
As stated above judges cannot become members of the House of Commons under the House of Commons Disqualification Act .
The Law Lords (the members of the highest court in the land), sit in the House of Lords which is part of the legislature, but by convention they do not participate in party political disputes.
Sub Judice rule – MPs cannot raise court proceedings in debate. The legislature cannot tell the judiciary how to decide a case.
To protect their independence it is extremely difficult for the legislature to dismiss a judge. S11 (3) Supreme Court Act  judges of the High Court and above who hold office during good behaviour are subject to a power of removal by the queen on an address presented by both houses of Parliament.
The judiciary accept the supremacy of Parliament – that Parliament can make any law it wants, but it insists that it has the right to interpret its meaning.
It is said that judges legislate when they decide cases and create precedent. However they will give preference to statute over case law (War Damage Act 1965 / Burmah Oil v Lord Advocate) showing that Parliament is supreme. Also the doctrine of precedent, expressed in the words stare decisis limits the discretion of the court as they have to apply the rulings of the higher courts. In Shaw v DPP, it was said that judges interpret and apply the law they do not create it.
Unlike in the USA, the judiciary cannot declare primary legislation (Acts of Parliament) unconstitutional, but they can review secondary (delegated) legislation.
The UK is becoming increasingly concerned with the Separation of Powers, particularly with Article 6 of the ECHR – The Constitutional Reform Act 2005 reforms the office of Lord Chancellor and the Law Lords will stop being in the legislature and have their own Supreme Court away from Parliament. It is trying to ensure the independence of the judiciary. But a full separation of powers is very unlikely as that would require an executive completely separate from the legislature and a new way of electing a Prime Minister, the UK is not ready for that.
The UK does have a kind of Separation of Powers, but unlike the United States it is informal. Blackstone’s theory of “mixed government" with checks and balances is more relevant to the UK.
It could be said that Judicial Review is the Separation of Powers working at its best – The JUDICIARY ensure that the EXECUTIVE do not exceed the powers that Parliament has given them, thereby upholding the will of the LEGISLATURE.
The separation of powers is not an absolute or predominant feature of the UK constitution. The three branches are not formally separated and continue to have significant overlap. However it is a concept firmly rooted in constitutional thought. It allows the judiciary to remain independent and to refrain from matters more appropriately left to the executive or legislature. Especially relating to prerogative powers and Parliamentary privilege. While the doctrine is not always respected it remains an influential body of thought that ought not to be “lightly dismissed" (Munro).