HMRC say…
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)
https://www.gov.uk/guidance/tour-operators-margin-scheme-for-vat-notice-7095#sect-4
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