Fifth, there is a two-party system which keeps the party in power continually on its guard lest the Opposition should make political capital out of just one of its tumbles on policy; sixth and last, there is a culture of democracy that preceded formal establishment of democratic institutions.
Parenthetically, checks and balances are a lot more than mere separation of powers: the essence of the latter is autonomy for every branch, which can only ensure that one branch of the government does not poach on the particular preserves of endeavour of any other. But checks and balances ensure that no branch of the Constitution lets another overreach its powers. For example, parliament makes laws and the judiciary interprets them in the light of the Constitution, even though in the case of Duport Steel versus Sirs in 1980 the judges said "it is parliament's opinion on these matters that is paramount". Interestingly, under the British Constitution judges cannot be removed from office except by impeachment which has been very rare. Also, the executive is discouraged from criticising judicial decision.
The US President is often called the world's most powerful functionary. However, the federal structure of the USA curbs his powers. That, at least in theory, is not the case with the British Prime Minister. The powers of Congress and the Supreme Court balance those that a President might exercise; the US Constitution lays down what the President can and cannot do, and the codified document can be changed only by the Supreme Court. Such constraints do not obtain in the UK; the general powers exercised by a British Prime Minister include that to appoint, reshuffle or dismiss Cabinet ministers, create peers in the House of Lords, give out honours, appoint ambassadors, top civil servants, bishops and judges, determine government business and Cabinet discussions and agendas, withhold information from parliament deemed necessary, use the media via a lobby system, terminate the life of a government and call a general election. Thus, the British Prime Minister would appear to have abundant powers. . The Prime Minister's position as leader of the majority party in the House of Commons, together with his position as head of government, thus combining legislative and executive powers, apparently amount to immense accretion of power. Additionally, many of the Prime Minister's powers derive from the prerogative powers of the Monarch. These extensive powers are wielded independently of parliament, and effectively give every Prime Minister the powers of a head of State, which include being in charge of the armed forces and security services, negotiating treaties. All this suggests that the Prime Minister can act like a virtual autocrat.
However, this is not so, as there are constraints on his/her powers. For instance, he/she cannot afford to be seen to surround himself/herself with "nodding donkeys", as a political commentator put it. The party he/she leads will not