Having this exact conversation with a place I've booked with over Christmas. The difference is about £500. So far the company letting the property have said they have not worked out what to do. I've paid about a 30% deposit but no invoice has yet been issued.
The have so far said that they have not given it much consideration but believe the relief has been introduced to support businesses hit by COVID. My point is that it is to ensure that during a recession they lower pricing helps ensure full occupancy by reducing the cost to the customer. I was quoted £3800k inclusive of VAT at the time of booking, if they do not pass on the VAT they are in effect changing the price offered after taking a deposit. My logic is that if VAT had been increased the owner would certainly not absorb this and my invoice in November would be at the new higher rate. Frankly this is the position they will be in when they take bookings later this year for next summer and the rate is back to normal.