Where do you go at the moment? Im in Hull so sometimes go to the Maidenhead in York which seems like a good shop but never got anything from there. But thats actually quite far when I look on a map lol - if you dont mind a bit of a drive Chilton Aquatics to the north of you has a great reputation for getting in good fish and a good rep for clean tanks as well. The North West has loads of good shops that are worth a drive particularly around Manchester not fair for us though!
I think Chilton will be worth a bit of a drive they have upto date stock lists on Aquarist Classifieds so you can see whats in before you go. What kind of thing are you after?
There's a Maidenhead Aquatics in Strikes at Stokesley. It's not as big as the one at York but it's nearer to Northallerton.
Paddock Farm used be good, but since Mark left it doesn't have the variety of fish it used to have (Mark is now at Petmania in Old Billingham, though that branch has mainly bread and butter tropicals as it also stocks marine fish).
Chilton Aquatics does have a good variety of the smaller S American cichlids.
Teesside Koi and Aquatics in Stockton. Don't be fooled by the name
There is also Tyne-Valley aquatics which I know about, they have multiple YouTube videos under the user of 'Pond Guru' They offer some pretty good advice too Their website also has a full price list on all of their fish Very handy
A bit far from you but try Ferrybride aquatics. They've a website if you want a look. Good quality shop and really good service. Won quite a number of awards too.