For High quality plants I would go for tropica plants - probably from Richard at aqua essentials. If I was feeling brave - apparently Pets @ Home stores stock tropica plants. I would only check them out on delivery day - they aren't kept in the best of condition.
For general fish equipment I tend to use Aquatics Online who have great prices and deliver promptly.
For Unusual mosses and cheap CO2 glassware I use Mike at Aquatic Magic an ebay store based in Malaysia.
For delivery of unusual fish types that I cant seem to track down in other places, I always try Monty at Tri Mar aquaria
Birstall Garden centre ( Aquatics ) have a good range of fishkeeping products, but the real attraction (IMO) is the number of Tropica plants they have in stock... I have never shopped there, but people say that the plants are of a good quality as well.