Personally I wouldn't buy on line I travel about an hour and a half to sweet Knowles aquatic in Staffordshire purely because there such good live stock you can see the quarantine tanks they wont sell for at least 2 weeks after arrival there fish are all quiet small so a good chance of them being young ones and there is never a dead fish in the tanks and so clean and most of all there truly fish loving people that own the shop it's a old barn on a farm
You had my hopes up for a great LFS in Staffordshire. Sweet Knowles is near Stratford (Warwickshire) and on my list to visit but just a little too far to be my local.... This is the one in a farm right?
It's a hard question to answer as your going into buying the fish blind. I'd look for reviews and maybe try a small trial order if I was enclosed to order fish by post. The postage cost wouldn't be my first consideration as they should all be similar. The only real consideration I'd have was if they had something I really wanted but couldn't get locally, if it was just neon tetras or the like I don't see much sense in buying on the net.