I also live in Manchester and have been collecting for about 4 years. Forbidden Planet in the city centre does stock some Minimates, but as was stated can be very hit and miss. I tend to buy mine from ebay or end up ordering from the States.
Upstairs in the Manchester store (last time I was there a few weeks ago) they had movie-related 'mates; 24, Star Trek, Back to the Future. Downstairs are the comic-related 'mates and they had Dark Avengers #2, Thunderbolts and Punisher box sets, FA Avengers and some leftovers from previous waves such as Iron Man movie, Havok/Polaris. Box sets tend to retail at around £12.99. Although they do eventually get shipments, they are ages after the 'mates are first released and aren't very predictable or reliable.
Happy hunting!