Currently, the monarch of the UK is considered a figurehead, meaning she has little de facto political power. When did the UK monarch first become a figurehead?
Always room to say more, but a very similar question came up here a few months ago and attracted some excellent answers:
Which British monarch was the last one to have "real power?"
A slightly different, legalistic, point of view is provided by
How and when did the British monarchs lose the rest of their executive and legislative power?