I think ! that maybe the Ionics new brush at somewher about 300gm may be interesting, but if you want light, just buy a basic floorbrush Bentley Addis Tesco etc they are about 8-10oz and adapt it. They are also really cheap. I suppose it depends on what sort of work you want to do.
But if you want more of an industrial type brushes with good scrubbing power at £2 then look at the "Home Style" D line and Nexus 11.5" . One has a mixture of monofilament and flocked bristle but as they have slightly longer bristles its good its interesting to give it a haircut and take off 1/2" of tyhe flocked tops with garden shears, leaving mostly mono bristles. It also has some sort of foam centre and weighs about the same as an Addis Bentley. These brushes have a removeable blue cap fitted and I suspect they are also branded for other store chains.