Maybe read up on how it works before rejecting it out of hand?
Britain has a bicameral parliament, with the leader of the majority of the lower house forming the executive, but the head of state retains the constitutional ability to dissolve parliament and order new elections if the current government is unable to function.
Having an apolitical head of state might be worth looking into.
The only people who can possibly think the British empire was centralised are those who have never opened a history book about it.
You obviously haven't opened a history book either, but at the very least, go to Wikipedia before making confident statements.
https://en.wikipedia.org/wiki/Scottish_Parliament
Subtle hint - 1707 to 1999.
The “global empire” was systematically disenfranchised under that model, which is a big reason why it broke up, and that was specifically called out by the US when it left.
I mean, there are arguably good examples of parliamentary democracy working at significant scale in a state, but the UK’s government at home while the empire was managed through a bunch of other systems is very much not one of them.