What is a VAT registration number?
A VAT registration number is alphanumeric and consists of up to 15 characters. The first two letters indicate the respective member state, for example DE for Germany.
When entering your VAT number, it must include the two letters that identify your EU member state (e.g. DK for Denmark, EL for Greece, and GB for the United Kingdom).
How is VAT calculated?
VAT will be calculated In accordance with the VAT regulations in force at the time.
http://www.hmrc.gov.uk/index.htm
Incorrect VAT information
If, as a result of the Customer giving BSI incorrect information, a sale that should have been inclusive of VAT is made without VAT being charged, BSI reserves the right to recover a sum from the Customer equal to the VAT that should have been charged.