I've used BES, Plumb nation, BHL a fair times, and Mr Central heating once.
Never had an issue with one of them. Though the last boiler I had from BHL looked as though someone had dropped the box, it was fairly mashed up. Boiler was for my house, so it went in!!
I've always used them for the cost saving or if I was working away from home and it was easier to have someone deliver an item than me drive it all the way there. Mr Central Heating I was forced to use as the customer specified I get the radiators from them. Pillar rads. All good, and a fair price.
BES have such a broad range that sometimes you have to go to them. No one in Tunbridge Wells stocks Armaflex for example. I don't use much of it, so I get it from them when I do.