The best books for the Europe 400-1200 course are probably
R Collins, Early Medieval Europe for the start, and then
R Southern, The Making of the Middle Ages and
R Bartlett, The Making of Europe for the later period.
For Britain 400-1400, try
A Williams, Kingship and Government in Pre-Conquest England for the early period and
D Carpenter, The Struggle for Mastery for the later. David Carpenter is one of the main lecturers for the course, so you'll get to meet him soon!
They're all big academic books, priced around £15-20, so you may not want to buy them, and I'm not sure they'll be available in local libraries. The KCL library and the University of Library will have several copies of each; you may want to go grab some in Freshers' Fortnight and have a read then? But I wouldn't worry too much about doing preparation for the course: the tutors don't expect you to have any previous knowledge.
EDIT:
Forgot to say, you won't be able to take both courses in your first year. To make sure you have a broad base of knowledge, the department don't let you take courses which overlap in period. If you want to take as much medieval history as possible, you'd be best off doing the Europe courses, 400-1200 and 1200-1500. Let me know if you'd like some reading suggestions for the latter.