The tax bands for alcohol contents in beer have changed recently. Basically, these adjustment in ABV help breweries actually make money again. Too many breweries have already folded this year, so this is actually a good thing. https://www.gov.uk/government/publications/reform-of-the-alcohol-duty-system/reform-of-alcohol-duty-rates-and-reliefs

Also, British beer culture has favoured lower-alcohol beers in the 3-4% ABV range for quite some time now.

Edit: also... it's just the alcohol content that is getting lowered, the beer itself will remain as flavourful as before. And there's another aspect: the category of non-alcohol and low-alcohol beers has become incredibly more popular in the UK in the last 2 years or so.