Ive used Trimar for a few fish that i havent been able to find locally and have always been impressed with the quallity of the fish and service, but i must say i wouldnt buy commonly available fish like discus via mail order unless it worked out substancially cheaper including with the delivery charge, im not even sure that Trimar stocks discus anymore since expanding the catfish and normal CA and SA cichlid selections.
There are a lot of very good shops around London if you are prepared to travel a bit, i thoroughly recomened a trip to Crewes Hill in Enfield where there is a cluster of shops all very close together which hold a amazing selection of fish between them, Wildwoods and Fish Biz are two worthy of particular mention out of the 5 or 6 shops there. Another shop worth a visit is Wholesale Tropicals at Bethnal Green which always has a good selection of some of the less commonly seen fish.