Why is the alcohol content in British beer so weak?
It's the points/levels in the duty imposed that have made UK beer weak, from a historic point of view.

Over 4% and you are on to a higher tariff point for the duty paid on it, so brewers aimed at 3.9x% for a long time, although there have been changes, and a lot of brewers find that people are prepared to pay a few pennies extra for a stronger brew these days.

Pub prices were once highly sensitive to duty, but so much of the price of a pint is now determined by the running costs of the pub, rather than duty, that stronger beers are becoming more common.

When I was a lad, Old Peculier was only sold in 6oz bottles, and a lot of pubs would only sell it in halves on draught ...