His acts are a criminal act within the UK as well. If it wasn't, I would see no reason whatsoever to not extradite him indeed.
This is a basic requirement of extradition generally. I seem to recall we used to have specific problems in the UK with something that the Americans call (IIRC) wire fraud (?wire something, anyway) which was an offence in the US but not here at the time, due to peculiarities in the Theft Acts. The long and the short of it was we didn't extradite people for wire fraud until we had amended our own criminal law to catch up.

I didn't know the German position, although I did know other countries took the same approach (You might remember ex-president Alberto Fujimori suddenly rememberd he had Japanese citizenship and fled to Japan when it looked as if Peru would indite him for corruption).

Given that no state could afford to deny a territorial basis of jurisdiction (since that would mean that Germans were perfectly free to commit crimes in the UK and we would have to leave it to the German state to deal with them) I would have thought that the personal approach should give way in time, since they clearly clash. What happens if a German commits a murder in, say, Morroco, and makes it back to Germany before being arrested? Is he really not extradited? Do you try him in Germany instead? if so, according to the law of Germany or of Morroco? Do you fly the witnesses over?