Well the first subjects in the Bristol Groundschool module 1 are -
Mass & Balance
Instrumentation
Flight Planning
General Navigation
Meteorology
Human Performance
VFR Comms
IFR Comms
Princess PP - hHich subjects are difficult really is a personal question. For me it's Instruments. Could be totally different for you.
Will have my first proper look at the Bristol course this weekend, but from the quck scan i've had it looks very professional. Comes with a nice Jepp binder with all the approach plates/hi-lo charts too, although I think all schools provide this, no?