Perhaps you could do something on how Britain or British politics has changed from 1900-2000/1800-1900. For the 1800-1900 period you could also look at the Industrial Revolution. Depends what you're into really. There's a lot of 'movements' (like the Industrial Revolution) which you could focus on, but there's also a lot of 100+ year wars you could do if you're more into battles n such.