I just had a look and I see the UK now has far higher taxes on beer than Denmark, about four times the rate, which if it's on an alcohol percentage basis (as it is in the EU) will be magnified for the strong Belgian brews. I don't know if UK still has alcohol percentage bands - historically they defined e.g. the difference between a best bitter and a premium ale - think there was a boundary at around 4% set around the 2nd WW, but my history is a little ropey
Excise duty on 330ml 5%:
UK: € 0.4187 (36p)
DK: € 0.1078 (9p)
If there aren't alcohol percentage bands and it's just pure rate, then that would be around 72p on a bottle of Chimay Blue in UK, and only 18p in Denmark.
Given it sells at about £3,70 here in supermarkets (with five for 100kr - which is currently £2.32 per bottle) that doesn't seem to account for anything like the price difference I see.
A quick check shows supermarket prices are Duvel for £3.60, a 75cl Westmalle Dubbel for £8,13, so maybe Chimay was a special case there? At £4.90 it was
So is there a specific higher tax rate for strong beers in the UK?