grocery gift cards?

[watersword]

Does anyone know of a way to pre-pay for groceries, preferably online? I have a grant to cover expenses while I am traveling in the UK in August (yay research fellowships!), and they want receipts, understandably. I'm not thrilled at the thought of collecting a ton of little pieces of paper from Tesco's and Sainsbury's to turn into my bursar's office, and if I could just buy, like, a gift card before I go, that would be awesome. My Google searches are not helpful, but I have the sense the websites aren't, shall we say, au courant with reality.