I don't know where you get the 80% figure from.
Avgas is subject to excise duty at about 30p/litre, and motor fuel at around 45p/litre (from memory). Both are then subject to VAT at 17.5%.
So a 1.65 litre of Avgas would be about 1.05 if untaxed.
Here in the US, the cost is typically around 80p/litre in UK terms ($5/USGallon) though it's a bigger market with smaller margins compared to UK/Europe.
£1.95/litre is rather at the extreme end of the UK spectrum.