Hello Leona,
If you proceed through to the end of the ordering process 'Rechnung and Lieferung' (Invoice and Delivery) you have the choice of either Barzahlung (cash), but only when you collect the certificate in person 'Die Urkunde wird abgeholt' or Ueberweisung (money transfer from a German bank account), if you wantthe cert sent to you.
I think as Peonie suggests, you should email them. They should be able to provide banking details (IBAN and IBAC) to allow you to do a money transfer from a UK bank.
Justin