It would be far better to use a LFS, spend the £12 on fuel to drive to a decent one if you have none near by. If not then Kesgrave tropicals on ebay always get good reviews.
Also check out the classifieds on here, Gumtree and Aquarist-Classifieds.co.uk
All online merchants charge for delivery, so I second what Minnnt said about spending that money on petrol instead, unless you have mobility issues.
Have a look at this, there may be somewhere near you.
If you're right the way down the far end of Cornwall, then Trimar in Truro seem to be pretty good, having spoken to their people a couple of times on rare livebearers.
Other than that, this link is a store-finder for Maidenhead Aquatics. For a big national chain, some of their branches are pretty good.
i recommend STE1200 on ebay, he's on some of these forums and all my fish he posted to me were healthy and still alive 6mnths later. He specialises in south american cichlids...