BA Modern History - Then just pick topics relevant
A word of warning - you will almost certainly find that in the space of 3 years you will either want to do something other than imperialism, or that the study of it is actually very different to what you imagined. My advice is not to pick a degree because you like a part of it. Even with History, the most flexible degree course, you will have to do other stuff. Make sure you do History because you like history, or ppe because you like ppe, and not just because you like imperialism. Snap decisions are very frequently bad ones.
Nb. read up on Rome, Persia and the Chinese - it should give you a better sense of whether you are interested in Empire as a concept or just in the European vision of it
If you wanted to do history degree tailored to imperialism though here is what your finals papers would be:
British paper -- after 1500 will have a few questions on Imperialism. Ideally the 19th century paper.
General paper --offers 'Imperial and Global history 1750-1914'' aka Empire paper, where you can do either Africa, South Asia, Americas, S.East Asia or Britain's 'white' colonies : SAfrica, Oz, NZ and Canada
Further Subject - 'Imperialism and Nationalism 1830-1980'
Special Subject - 'India: contesting the nation 1910-1950'
Thesis - You get to pick the topic
Disciplines - Always has a few empire related questions, as well as a few on race