If I were going on holiday from the UK and needed to change sterling to euros one consideration would be Travelex UK, they don't charge commission, and will send me the money via mail to my home address (alternatively I can pick up at the airport if I desire).
Can you not use the German arm of the company ?
My second choice option:
Alternatively I would purchase before travelling a Prepaid Travel Money MasterCard® from Thomson Holidays UK (part of the TUI group).
Just load up the card with the amount you want & there are no commission charges or purchase fees when you buy or reload your card*, the exchange rate is also fixed at the time of purchase, so you don’t need to worry about fluctuating rates and the uncertainty of credit/debit card charges on returning.
(*2% load fee applies to Sterling cards and subject to a minimum £5 application fee).
(Obviously similar pre-paid travel cards must exist in Germany & if my German was of a reasonable standard I could have given an example for you, but i'm sure if you go to http://www.tui.com/ you can find their own branded Pre Paid Travel Card)