Fix UK VAT lookups

[Mollemoll]

UK VAT Lookups are no longer accessible through a public HMRC api. The requester needs to be registered with HMRC to acquire a key that can be used to perform lookups.

For more information see https://developer.service.hmrc.gov.uk/api-documentation/docs/api/service/vat-registered-companies-api/2.0

[vatfix]

Since Brexit, GB VAT numbers are no longer available via VIES. The official way to validate them is through the HMRC “Check a UK VAT number” API, which requires an API key. For Northern Ireland (XI), validation still works via VIES.
A good approach is to implement logic that sends GB numbers to HMRC’s API and all other EU VAT numbers (including XI) to VIES.