Buying from Play store using a credit card with foreign billing address
As far as I know Google doesn't allow the use of International cards (at the moment).

I also heard from some other people (and t0mm13b acknowledges it too apparently) that the Play Store looks at your current SIM card to know your home country. So maybe if you could get your hands on a German SIM card (maybe an old one that doesn't really work anymore), you can still pay for applications.

Or you can have a look for some cheap credit card in the UK.