You must register your business for VAT with HM Revenue and Customs (HMRC) if its VAT taxable turnover is more than £85,000.
You can of course voluntarily register below the threshold
However, there are special rules for TOMS which mean instead of Turnover the threshold is based on margin. This can make a massive difference as it takes a lot longer for your margin to hit £85,000!
Tour Operators Margin Scheme (VAT Notice 709/5)
4.1 What taxable turnover is for VAT registration or de-registration purposes
If you’re considering whether you must register for VAT, or whether you may de-register, your taxable turnover is regarded as the total of:
total margin on your taxable (including zero-rated) Margin Scheme supplies
full value of:
taxable (including zero-rated) in-house supplies
taxable agency commission
any other taxable (including zero-rated) supplies you make in the UK