Last time I was in Germany I bought a prepaid Vodafone CallYa Basic SIM card. My cellphone is unlocked - so everything works seamlessly. Rate varies, but here's a good website to check for current prepaid offers for many countries:
http://www.prepaidgsm.net
Unfortunately I don't know of any SIM cards that will give you the most values in both UK
and Germany - you'll probably do better if you buy one for each specific country. Check out O2 as well - heard great things about it.
Hope this helps!
Jamester