It would be hard to stick to one supplier. I've used several as some stock products others do not offer; there are times when one of them might be temporarily out of stock of an item but others have it in stock; and there is sometimes a 'worthwhile' difference in prices for the same products. I've used SKS, Aldridge, Duffells and Tradelocks mainly. For uPVC Greenfox is often recommended although I haven't used them myself yet. But there are many others including specialist suppliers for specific products or tools.
I've used Green fox for over a year now and can't fault the service, they are normaly best on price and good at helping identify the odd ball upvc locks. Duffells are good depending on you get through to on the sales team, sks also very good and helpful, aldridge is good but only if you know what you want i guess this is becuase the people in the call centre aren't locksmiths. Nigel Rose marketing services are also good and local to me which helps.