They aren't at all rare. But they do tend to be seasonal. In any case, visiting Tropical Fish Finder, which databases stock in UK fish stores, I
discovered that at least two branches of Maidenhead Aquatics as well as Wildwoods have it in stock at the moment. Wildwoods does mail order, but it is always worth an expedition out there just to see their amazing stock. Technically they're in Middlesex, Enfield to be precise, but right on the eastern edge of the district.
Do bear in mind my comments repeated elsewhere about this fish (including in this month's PFK) -- while they *can* work in community tanks, they will work only with very specific tankmates: fast barbs, tetras, etc. They may not be as aggressive as other puffers, and certainly far less dangerous to other fish, but the can and will nip the fins of slow-moving or stupid tankmates. Gouramis, angels,
Corydoras, livebearers, etc would all be fair game! Also remember they are sociable, and you want to keep at least two specimens, and ideally three or more. They never look happy kept singly, being best described as "neurotic" or "hyperactive". Since you'll need a fair sized tank for three specimens (at least 30 gallons) I don't recommend this species for small aquaria, despite its modest size.
Cheers, Neale