I very much doubt they'll fit you. There is a way to tape two together, but to be honest, it's a waste of your time.
My only suggestion that you can buy in a store in the UK is Boot Staydry Plus. They're pretty poor, but better than baby nappies. I live in a large-ish city, and that's all I've found. I buy mine online now. If you live near a medical supply store, you might be able to do better, but I wouldn't count on it. I think the NHS provides such things for people who need them for a medical reason, which makes it harder to get them in high street stores.
Online, I can highly recommend BlushingBuyer for reasonable prices, and for fast and discreet shipping. Dorset Nursing sells cases, but they ship them in original boxes and if you look you can see the Molicare logo (for instance) under the tape. I was not pleased when I discovered this after they got shipped to a flat I shared with people. I know you want in store only, but you're pretty well screwed in the UK except online (although apparently Tena is sold in some places, but mostly these aren't ideal for men). Why can't you order online? You should be able to get a bank account and then set up Paypal or get a Maestro or Visa Electron, which you can use for online purchases. Or see if someone in the UK could mail you some if you sent them money via Paypal or a postal order. I'd offer to help, but I'm in the states until September.
Good luck!