Registering for a mobile phone :
Germany
Registering for a mobile phone
To get a mobile phone contract in Germany you must have residency. You must prove it by showing them your passport, and residency documents at the time of purchase. Be aware that the minimum contract period is usually 2 years. Check your options before signing up for anything as prices and offers change as often as the weather!
As for your UK mobile. Our guess is that you will be returning to the UK from time to time and may want a low cost way for people to contact you when you are there. So why not cancel your contract, but switch to a "Pay as you go"? This is a number that most of your friends/family will already have, so record your voicemail explaining that this phone is now only on when you are in the UK, and tell them how to contact you when you're not. Be careful when choosing your PAYG plan as some expect you to use your phone every few weeks to keep it active.