Buckman, I appreciate that point you make, but the contract is made at the point the terms are agreed with the VAT rate applying and immediately any payment is made . That payment is the requirement for a valid contract. As the VAT rate when the deposit is paid is the relevant one, the date of final completion is not relevant.
This explains why the C&CC are having to charge 20% on bookings made before the change, but the CAMH who have not taken deposits, can charge can charge 5%, as without a deposit there is no valid contract.
It could be possible for sites to cancel the original booking, return the deposit, and then re book with a new deposit, to get round this, if they have the time and inclination. If seen to do this on a large scale though, the authorities might regard this as tax avoidance.