From the UK Customs and Excise site linked to by Davebhoy, I find that from midnight on 13 April 2003 the tax rate per hectolitre per cent of alcohol is £12.22.

A hectolitre is 100 litres which is approximately 22 gallons = 176 pints. So duty per pint per cent alcohol is 6.94p.

So the Tesco Value bitter accrues only 14.6p tax per pint, and most draught bitters would accrue between 20.8p (3%) and 34.7p (5%).

So I would think that the price to the customer is more to do with profit margins made by the breweries and pubs, compared with those made by Tescos.

BTW I was told by the brewer on a brewery visit when I was on Long Island, New York last year that tax there is not related to percentage alcohol, but levied at a flat rate for all beers- so they could make the beer any strength they liked. There were all a lot stronger than our (UK) normal session beers.