Realistically there is almost no separation between executive and legislature; the PM can order both what to do because they're the same party. And these days it's run from the Cabinet office.

The only limitation is the semi-independence of the civil service, which Cummings intends to erase, and the possibility of judicial review of really bad decisions, which may be curtailed soon.

This isn't so much of a problem in the UK, where separation of powers is much weaker and the executive have a nearly-built-in legislative majority.

Of course, that does bring other problems...