My go-to supplier for stock is
Maidenhead Aquatics, the Cambridge branch inside Scotsdale Garden Centre.
Unfortunately, MA is a franchise so quality can vary, store-by-store, but I find this branch to have a good selection of healthy looking fish, well presented and backed up by knowledgeable, helpful, staff.
I've tried other suppliers, there's another MA nearby, a Pets@Home and a couple of independents too.
Never tried P@H but stock from some of the others has let me down with, relatively early deaths and, in one case, introduction of White Spot that took out existing stock as well. I know I should have a quarantine tank, but well, I just haven't.
Value for money is not just about price. You have to take account of longevity as well. In my limited, 3-years into to hobby, experience, cheap fish are generally not as good quality/longevity as those I'd rather spend a few pence more on.
Why do I go back? All of the above.
For non-fish supplies I rely on Amazon to source my SeaChem Prime, Stability & Pristine only when there's a 30% Extra "Free" offer on.
I'm a big fan of NT Labs for their liquid test reagents and Pro-F Pellet ranges, Tropical & Catfish. They're competitively priced and last ages.