VAT is at Invoice time so when you pay your bill in full. Ie when you pick it up.
Build week 10, then work on getting the car 13th week of the year. So April.
But theres an offer on VW website that if you order in October or November they will cover the VAT rise as long as the cars registered before 28th feb but if the build week you have is week 10 then fat chance of that happening!

Hold up,
Theres a few things been said why this is, going about.
Theres a shortage of RCD310(standard CD unit in the UK) and RCD510 (optional 6cd one) radios adding around 4 weeks min to the build weeks at the factory.
Theres a rumour that VW UK only booked a certain amount of build slots at the factory at the start of the year and basically its not enough for what they have ordered, they say because of the demand due to scrappage etc.