The VAT rate applicable to a transaction is dependent on the date of supply and the VAT rate in force at the time.
Transactions where a valid VAT invoice has been raised between 1 October 2021 and 31 March 2022 will be subject to VAT at 12.5% even if the time of supply is post 1 April.
If no VAT invoice is raised and just a deposit is paid then if the holiday is post 1 April the whole value will be subject to 20% VAT.
Please see the attached link to a more detailed article with examples covering most scenarios: https://www.eqaccountants.co.uk/updated-guidance-for-holiday-accommodation-providers-regarding-the-reduced-vat/