Perhaps a technical point but we don't have 'parliamentary government'. The House of Commons is not, and never has been 'the government' (ie the executive). It is a check on what the government can do, as is the Lords - the government cannot change the law, raise taxes or raise a standing army without their consent.

Although its not a phrase associated with Westminster (its more closely associated with former Dominions when they became self governing) it might be better described as 'responsible government' ie the government is held responsible by Parliament.

This tension between Parliament and the Crown has been a part of the UK's (and before that, England's) constitutional settlement since Magna Carta in 1215, and arguably before that.

Differing technical vocabularies, I think: I meant Parliamentary as opposed to presidential or semi-presidential (the two other main categories of Western-style democracy in political science). Americanists in particular see our insistence that Parliament is not part of government quite bizarre - they include the judicial branch too.

You're absolutely right that Parliament hasn't governed directly for a long time (arguably not since the civil war, and that was itself a highly unusual period where some questionable constitutional innovations happened). It's also the case that the question of whether the Crown can govern without constraint from Parliament has been settled in principle for centuries: it can't. The Commons has had to repeatedly assert this over the years, and it looks like they're going to do so again.