It is the most odd requirement. It just has to be a recognised UK VAT number and it seemed so long as it had some relevance to you or the craft that was fine.
I would think if you have a UK company and are registered for VAT that would be the solution. It makes no sense though as you pay VAT on the fuel. It seemed there was a box on the computer that needed filling in a without it was a no go. And no other solution.