That would qualify as a price increase by HMRC, not by Ineos.Have you tried telling them you are cancelling under this clause:
"6.4 The Customer may terminate this Agreement within 14 days of being notified of a price increase by INEOS, in which case it will receive a full refund of the Deposit paid."
Their email "UK Vehicles Tax Update" [dated in my case 6th April 2023] counts as a price increase.