There are a number of suppliers of Sake in the UK. Here are a couple:
Tengu Sake:International Wine Challenge Innovator of the year 2016 imports good Sake from a select group of Japanese breweries
Or you could try Japan Gourmet:
At present many supermarkets try and and sell Sake. Sainsbury’s sells Doragon sake, which is made in the Netherlands. They used to sell Hakutsuru which is fairly decent but have now stopped. They’ve written an article on Sake which is good:
Waitrose on the other hand does Sawanotsuru again not bad. Tescos sells Choya and Doragon as well. Most of these are what are called ‘Futsushu’ or basically table wine.
Sake Samurai has a further list here : http://www.sakesamurai.co.uk/where-to-buy-sake/