There are at least 3 bands of tax on beer, one for below 2.%8 ABV, one for 2.8+ to 7.5% ABV and one for higher than 7.5% ABV.
The price you pay often has very little to do with taxation. A 2.8% beer may cost the same as a 6% beer in many pubs.
Possibly the reason for poor sales of low alcohol beer is that many have been dumbed down to remove any flavour.