In all honesty most of the retailers offer very little in competitive pricing. You may save a few pounds searching but not much. Unless your lucky and a sale comes along. Bradshaws is OK. I've also had good service from Richdon Koi, Boddington Koi and Koizone.
Others include Koibitz, water gardening direct, koi water gardens ltd (koicarp.org.uk), Nottingham koi company, ichiban koi, East Riding Koi (koicarp.net), dankoi. (some sites in their pricing exclude VAT and look cheaper but aren't)
You may get a good deal for a filter system by phoning up the companies. They often sell them for less than the stated price but don't advertise it. Most often with the Nexus filters which are rather pricey but excellent filters.
cheers