Build level 3 fortifications in all your provinces. Preferably, wait until Britain gets into a brutal war of attrition with somebody (they usually do), and wait till their war exhaustion climbs above 50%. Garrison each province with at least two or three artillery brigades and however many regulars you can mobilise - be very, very well dug in. That's the defensive tactic.
Alternatively, wait for them to be in said brutal war of attrition with someone else: they'll usually have very few soldiers in England itself. Take and garrison London, and they'll fold like a house of cards once their war exhaustion goes above 25%. You've got to be REALLY quick though, before their reserves get mobilised.
If you can, have a navy of ironclads at the point where Britain's navy is still wooden.
Alternatively, buy a colony from someone. Declare a colonial war with the UK, and keep soldiers in your one colony so it can't be lost. Bit gamey, but it has the desired effect.