You can get GF food on prescription from a normal pharmacy. You have to see your doc first and you get awarded points depending on age/sex/pregnant etc. Your local chemist/pharm will give you a very comprehensive list of the same foods as posters have listed here and the usual stuff from the big five supermarkets. You just select the amount that you want up to your points value. List it onto a big custom made prescription form and pay (if eligible) £7.65; the standard scrip charge. A couple of days later collect it from your pharm.
And have a look at Ocado´s GF range.