Factological amendment to Simon R’s Rough Trade bit: it depends on the RT shop, dunnit? Yes none of them stock garage but in general the Ladbroke Grove one is the one that goes for nu-skool breaks et al. The Covent Garden one is much better: minimal breaks section and yes they do stock dancehall. In fact last time I was there they were blasting one of the riddim albums (Hard Drive) – I asked what it was and was seized on with enthusiasm by the counter guy and inveigled into buying an import dancehall 7″ they’d just got in. (And could have left with plenty more if my wallet had stretched to his recommendations.) Their hip-hop section is broader too, though neither shop is very good on the UK stuff, ‘proper’ or otherwise.
I think the no-hardcore and no-garage thing isn’t entirely a class thing, too – I think it’s partly to do with the economic trajectories of these musics: both of them hit commercially big quite early on and then went underground, by which time the anti-pop radar of the Rough Trade record buyer had no doubt been firmly pinged. (Of course, they kept on stocking baggy/grunge/americana etc. no-hopers long after those commercial stars had waned, but as Simon says they know their market.)