there is Tuftyfluff Hamstery in Borehamwood but recently it seems no one's heard from Sue... if you manage to get in touch though, she has great quality hams, my sister has a Winter White dwarf from her! I think there's also a user I've seen here under londonhamsters.co.uk or something? they might have some litters now/soon.
I would personally not take a baby from any younger than 6 weeks, though some 'breeders' are selling them from as young as 3 weeks. 3 weeks is when they're weaned off mum and the babies are separated into gender cages, but I wouldn't feel comfortable taking one that young as they have just left mum, imagine taking them from their siblings at the same time!
at P@H they cost £9.50 but I've noticed that breeders often sell for less. I generally see them selling for between £5-£10 from decent breeders, and I probably wouldn't pay any more than £10 even if they are show and breeding quality, if simply because I'd only get hams as pets and nothing more, so there would be no point.
there is a National Hamster Council, which some breeders are associated with - these are definitely trustworthy. there are also some hobby breeders who haven't paid for the memberships but can still be reputable - you should find out if they can identify the colours of the pups in the litters, if they can show you pedigrees of the parents so they are not just breeding petshop hams, if they ask for the hamster back no matter what if you find you cannot take care of them anymore, and if they have a caresheet or good advice (so none of this "they can live in tiny Rotastack modular cages" and "buy 2 Syrians for a discount, don't worry they can live together because they're related" stuff) for you as a new hamster owner. personally, these are the criteria I would look out for in a good breeder. also try emailing breeders you find to see if they have pictures of the litters they might have (if they've taken any of baby hams before their eyes are open/under 2 weeks of age, I wouldn't trust them). I think these are all the things I would look for, but I might have missed some.
edit: actually I've just remembered that vanilla-yazoo on this here forum has a litter at the moment! they're still too young to open their eyes (any day now though!!) for any pictures, but she's based in London and I would trust her haha. they'd not be ready until mid-late June though, so if you're happy to wait.. look for Lucy & Bon Soir's litter in the Breeding forum for more info. I reeeeally want one of her ham babies, no joke.