>>106121258in the UK you can do it easily, most supermarkets and petrol stations carry sim cards. vouchers are harder to get hold of, only some supermarkets have them. but you can just buy online anyway
i was greatly surprised to find that I needed to present my passport to get a prepaid sim in (some of) the eu. shit is weird, doesnt even seem like theres a proper law. in poland the man at a phone kiosk said i needed ID, but then I got a free sim and added credit with voucher just fine.