UK businesses will be able to continue to use the EU VAT number validation service to check the validity of EU businesses, but UK VAT registrations will cease to be included.
The UK Gov has an API in beta for checking VAT numbers:
Following your feedback, we can confirm that the Check a UK VAT number API has been changed from an application-restricted API, to an open access API today.
As a result of Brexit, regrading VIES:
The UK Gov has an API in beta for checking VAT numbers:
https://developer.service.hmrc.gov.uk/api-documentation/docs/api/service/vat-registered-companies-api/1.0
(API access is controlled by credentials. Users must sign up for GOV UK Developer Hub)