For ME, the best place is the Devon Camera Centre (mainly because it's closest!), closely followed by Mifsuds (who aren't too far away). I can get to both so I can see for myself exactly what I'd be buying, although I'm very happy with the way Mifsuds describe their 2nd hand stock. In fact, I've been in the shop while someone's been on the phone, and the shop assistant's been examining the kit minutely, describing even tint scratches in the paint on a tripod collar for a Sigma lens (which one almost expects to be scratched from new!). Also heard very good things about Ffordes, although I've not got any personal experience of them.