I haven't checked but it's likely that if you looked on amazon if you have an account there you could just get it shipped. Or, if you don't use amazon, just google it and find a site that ships and looks legit. I don't have any specific reccomendations as I live outside UK.
Have you looked for B12 in health food stores in the UK? One thing to look for is a B12 supplement that is in the form of
methylcobalamin and not combalamin. It's easier for the body to absorb methylcobalamin.