B & Q apparently have more than 300 outlets scattered across the UK
whereas
Screwfix and Toolstation have only about 20 between them so they're not
really competing in the same market.

Indeed. I imagine that they can keep costs low by expecting each counter to
cover a much larger geographic area. If they had a branch in every tiny
market town, then prices would have to rise.

However, I don't care. There's one near me, so I'm alright!

Christian.