If you were going to consider the UK monarchy in it's most recent form, you'd go back to the Norman invasion of 1066 and King William the Conqueror.
Most of the big land owners in the UK to this day can be traced back to the Norman conquest.
If you were going to consider the UK in its modern parliamentary form, you'd go back to the English civil war (1642-1651) where Oliver Cromwell overthrew King Charles I.