Im starting a Primary PGCE in September, so dont know lots of detail on British History or medieval times, but I would start by visiting the library, googling, checking out bbc bitesize, for each topic and start drafting an outline of the topic. so for eg with British history, how far back are you going to cover? 100 years or 1000 years? lists of kings and queens, major battles, important figures, impact of disease (eg great plague) and events (great fire of london), industrial revolution, sanitisation, politics etc. If you have 3 months, then timetable your time well, start early and work hard! Good luck, sounds interesting.