In the arrivals area at London Heathrow there are prepaid sim card vending machines. I made the mistake of getting the data only sim for vodafone. Apparently it is only for tablets and will not work on phones for data only.
There is a vodafone store at Paddington Station that will sell the proper sim card that allows data and voice for 10 pounds.
When in Germany, you can go to a T-mobile store and get an Xtra prepaid card. It costs 10 Euros and gives you 10 Euros credit. Internet is unlimited at 1 Euro a day IIRC. Roaming is not cheap for the Xtra card.