I agree, proving your address with a letter is becoming increasingly difficult as everything goes online. Will your bank supply a printed statement (usually for a charge though)?
A service I’ve needed in the past and now think I will start, is providing UK gift cards and virtual items for Bitcoins. Items like prepaid Visa numbers, amazon.co.uk giftcards, Tesco giftcards and mobile phone tops ups. Are they any other items people would like supplying?