Hi,
If you provide services to a business customer in the UK then these would generally be accounted for by the UK business under the reverse charge procedure.
Please see the guidance below:
Place of supply of services (VAT Notice 741A) 5. Reverse charge
There is no definitive method to complete an invoice as an overseas customer to a UK business customer.
However best practicee would be to include your German VAT number on the invoice and a statement which clearly shows that the supply is subject to the reverse charge procedure by the business customer.
Thank you.