Legislature (parliament) and executive (central government)
Walter Bagehot:
The efficient secret of the English Constitution may be described as a close union, the nearly complete fusion of the executive and legislative powers. According to the traditional theory, as it exists in all the books, the goodness of our constitution consists in the entire separation of the legislative and executive authorities, but in truth, its merit consists in their singular approximation. The connecting link is the cabinet.
Commons Disqualification Act 1975, inter alia, prevents executives and judicial from holding a seat in Commons.
Barber: whilst there is a strong connection between the legislature and executive branches –with the political party of the government tying the two together – there are also profound differences. The executive branch in the United Kingdom is very large:..there are hundreds of thousands, perhaps even a million, people working in the executive. There are civil servants, soldiers, policemen, nurses, all of whom could be considered part of this branch of the state. They, for the most part, do not sit in parliament. Indeed in a clear instance of the separations of persons, civil servants are barred from sitting in the House of Commons. In their working life, these state employees are expected to be political.
(TLDR; functions are separated)
The executive holds political importance.
Ministers have the power to enact delegated or secondary legislation. Often in the form of statutory instruments, members of the government have the authority under various enabling Acts "to change the law to meet changing circumstances without the sledgehammer ( and the delay) of new primary legislation" these statutory instruments take a number of forms including "orders, regulations orders in council, schemes, rules, and codes of practice"
A notable mention of such delegated legislative powers can come in the form of what is known as Henry VIII clauses. these empower ministers to alter or amend primary legislation. Prominent in Brexit.
Powers to pass secondary legislation are therefore notable and give members of the executive the authority to act as legislators.
Such close fusion between legislative and executive authority in the UK constitution can be seen as a threat to the values espoused by montesquieu concerning individual protection from arbitrary use of power and and the principles at the heart of the rule of law.
Lord Chancellor and Lord Hailsham, identified these risks of "elective dictatorship" in October 1976. The considerable influence over the house of commons and the rest of parliament.
To counter this there are debates, discussions and parliamentary procedures which prevent arbitrary uses of power.
The most prominent way in which accountability exists is the parliamentary question Time. Along with select committees which "examine the expenditure, administration and the policy of the relevant government department and associated public bodies."