The rules for VAT registration are the same for self-employed people as well as limited companies.
The basic rule is if your sales is going to be more than £82,000 in the next 12 months you should register for VAT. You shouldn’t wait until you pass the £82,000 if your monthly run rate is higher than £6,833 i.e. if right from the beginning, you think you will go above annual £82,000, you should register straight away.
You can still register for VAT even if your sales don’t go above £82,000. The reason why you may do this is to appear to customers bigger than you are.