Crass and pretty much the whole Crass Records/Anarcho-Punk scene that grew around them. The Mob, Rudimentary Peni, Amebix, Poison Girls, Zounds, etc.
One band completely ignored is Flowers in the Dustbin, who've recently had a myspace made up:
http://www.myspace.com/fitd. The first song is one of their new ones, it's not bad but I would recommend the others first. Also, they've also put their entire discography up for download on their website:
http://www.flowersinthedustbin.co.uk, I would probably recommend "Freaks Run Wild in the Disco" as a starting point.