[pmyteh]

In theory Parliament controls everything, but in practice the executive controls everything (because they control Parliament). The separation of powers that does exist is largely because of 'leakage' in Ministers' control of events. They can straightforwardly change the law, for example, but you need to change it before you do the otherwise-unlawful thing, not afterward. If you don't, the courts will nullify your actions. You can dismiss 'independent' quango heads quite straightforwardly, but it's politically expensive (and somewhat time consuming) to do so too frequently. You control the Parliamentary timetable, but there are a few gaps in it for opposition day debates and private members' bills. That sort of thing.

We do have a stable and functional democracy, though it's rather brittle against a bad-faith executive. One prominent theory is the 'good chap' model of government, which holds basically that the system is set up such that 'reasonable chaps like us' can govern well and with few constraints, but the flipside is that Johnson's impact was limited by his government's lack of competence, not by institutional constraints. That might be liberating or terrifying, depending on your views about good government.