Hi Kate,
This is probably a good question to ask your accountant because the situation varies according to what your taxable VAT expenses are. A flat rate is better if you don't have many taxable VAT expenses, but if you have lots than you should just apply for VAT.
HMRC says you can register anytime, although anyone earning over £67,000 in the previous 12 months must register. Check out how to register for VAT here.
Moneymagpie :-)