Whatever happened to the other RON- UK SUL used to be 98 RON- where has the missing RON gone?

Here in France the SUL is all 98 RON- and it seems to be taxed the same as the normal stuff- currently is 1€ 5 cents at my local Shell garage- NUL is 1€ 4 cents. Is it really taxed that much more heavily in the UK to account for the price difference?