I doubt VAT will go up, no justification for it on a fiscal point of view.
Also with WGs its a cash transaction, they don't pay VAT, if for some weird chance that VAT does go up, your typical punt per hour is £120
This means the pre VAT rate is £100 + VAT at 20% = £120
If VAT went up to 25%, hypothetically, then £100 + VAT at 25% = £125 so not a greta deal.