The EU tool for checking that a VAT number is valid won't work post-Brexit.
There is an alternative!
With the UK leaving the EU’s VAT system from midnight on 31st December 2020, UK VAT numbers will no longer be maintained under the VAT Information Exchange System (VIES). This means you will no longer be able to check whether a UK VAT number is valid using the VIES tool. HMRC has published a new tool which will allow you to check a UK VAT number as you would have using the VIES tool. This will be updated in real time, and will also show the name and address of the business associated with the VAT number. You can find the new tool here. This will also provide evidence of when you checked a VAT number in case of a compliance visit. Note that the tool only applies to UK businesses, so you will still need to use VIES to check numbers for EU suppliers if you use them.