To check that a UK VAT number is valid you should first check that it is 9 digits long.
(This does not include the GB which is sometimes stated at the beginning).
The specifications of valid VAT numbers were originally gleaned from a variety of sources found on the web, but have since been confirmed using a VIES document.
All necessary check digit validation is performed except for the new style French TVA numbers, which are not thought to be yet in use.
When a business sets up a new purchase ledger account for example, it is good practice to check the supplier’s VAT number because, if it is invalid, so is the tax invoice and HMRC may disallow your input tax claim.
However, the fraudulent use of another trader’s number is prevalent in certain sectors of business, so checking is potentially a valuable precaution.
HMRC have recently issued a revised Notice 725, The Single Market, which includes a useful checklist, see table below.
For orders of downloadable software placed by customers in the European Union, the EU's Value Added Tax (VAT) may be collected on the order.
All VAT collected will be remitted to the proper taxation authorities in the EU.