Hey Chris,

I guess a good option for you would be Manifone.

You can stick with your current T-Mobile tariff, just make sure that
calls to UK landlines are cheap enough.

Just open an account at www.manifone.com/uk

They'll give you a London phone number that you can point to your
girlfriend's number in Germany.
Calls will cost 11p a minute to any german mobile + the cost of
calling a UK landline from your phone.

Works in a PAYG fashion, and is fairly simple to use once set up.
You'll actually get 8 UK numbers so you can set up more such "low cost
routes" to people you need to call abroad.

Hope this helps,

- Thomas