I live in Grimsby, used all of the fish shops you mentioned, the Maidenhead aquatics in Beverly is very good imo, staff are very knowledgable and where prepared to spend best part of an hour trying to catch a botia modesta in there maze of conected tanks lol, there is a fairly good aquatics section at silica lodge garden centre in S#37###horpe, they have a selection of 6ft plus tanks with Rays and specialist cichlids etc prices are very good to and they will order you anything if they do not stock it, Sixhills aquatics is the best place in Grimsby they are very helpfull and are willing to help you aquire any fish if you can keep it.
To the person above who took his severum to brigg garden centre, i was there at the weekend and the barbs seem to have caused it a bit of damage im afraid,
Steve