Products purchased inside of UK territory, which then are exported outside of UK borders. Must show a valid purchase order with relevant EU or other importation documentation, specific to country equipment will be advanced onto.
This is a HMRC requirement under VAT notice 703 relating to goods entering europe from UK mainland.
Therefore failure to produce the correct documentation will result in items being sold with VAT added @ 20% UK rate.
If documentation is not available, then the notice 703 on HMRC website states vat has to be added onto goods purchased within the UK mainland.