Looking for a solution for this myself, my guess is that the only solution is going with multi-companies, and setting each company for each fiscal position you need:

https://www.odoo.com/documentation/user/14.0/general/multi_companies/manage_multi_companies.html

Hello M,

You can create child contacts of the Main Contact and assign VAT to the child contact, then use that particular child contact wherever needed,

In more details:

Main Contact:

German Child Contact: with VAT 1234

UK Child Contact: with VAT 456

You can use the UK Contact for invoicing when you are dealing with the UK Business.

Hope this solves your problem!!

Thanks