I only moved across the pond a year ago so I don't really know what's what yet but for London maybe check out heatwavemagazine on instagram. Also the Another Subculture person posts on the see-saw forums, I don't get the impression that they book shows but maybe can put you in the right direction.
For Bristol maybe check out the Specialist Subject / shittyfuturespresents / bristolspop folks.
Sheffield also seems pretty happening - maybe check out deliciousclamstagram.
Brighton seems to be another hub - not sure who's booking there but I get advertised a lot of stuff at the Hope & Ruin.
Cambridge, where I'm based, has Crushing Death & Grief, which is a bit more on the avant/noisey side. There's not really a big DIY scene here.