If you purchase goods from a German company you must expect to pay them in Euro. A conversion from GBP will take place at some point, either at your bank or at Paypal and this is what you are seeing.
I recently purchased some goods from the UK and, again, Paypal was used. At one point I saw a message on the screen informing me that the goods cost X GBP and Y Euro would be deducted from my bank account. Here the system is operating in reverse and at no point would I expect a UK retailer to accept Euro funds.