You can use a Post office travel card, although one might think it is only for foreign currency, actually you *can* load sterling on it but they do charge a little extra for it, if you go into a branch to get one, you need to being ID, but weirdly enough, they don't ask for this if you order it online (though you need to pay for it by a cc, not sure if that might be a problem) I am going to order one and see if I can use an alias.
Also, it is chip and pin enabled so you can use it for small purchases also.