I would always advise anyone, however small the business, to use an accountant. There are so many things to consider with tax and VAT. You'll never be able to cover them yourself without a lot of research. I've always found that an accountant will save you far more than their fees.
When you register for VAT, you can indicate that your intention is to make enough taxable supplies to see registration.
In your set-up costs, some, but not all expenses will be re-claimable. This is where an accountant comes in.
After a year or so, if you look as though you're going to be below the threshold, then it's a simple procedure to de-register. Remember though, HMRC do not have to accept your reasons for de-registering, especially if they sniff that you're doing this merely to recoup your set-up VAT.
It's always a matter of intent, and following the "spirit" of the Laws with HMRC. It's not just a matter of following the rules, rather, how you interpret them.