"Long Good Friday" is a terrific film - Hoskins' best ever (one of Piers Brosnan's earliest roles, too). I prefer it to "Get Carter," but for two for the price of one - with some very nasty sleaze and violence - you might want to check out "Mona Lisa" with Hoskins AND Caine.
http://uk.youtube.com/watch?v=F8CAdm_1gZAAnd of course, the ultimate British cops and gangstas film is "The Sweeney" (a spin-off from the TV series). Teh second one - about the gang of armed robbers living on Malta with their families who go back to London for "one last job"which, naturally goes horribly pear-shaped is the best. Classic scene when the top villain goes fishing with a sawn-off Purdey.
Sweeney 1
http://uk.youtube.com/watch?v=rebjsci5VAY(Cheesy trailor, but you get the drift)
Sweeney 2
http://uk.youtube.com/watch?v=09luU0hovfIWorth tracking this film down.
Then there is Oliver Reed at his most brutal in "Sitting Target." Reed is a very, very violent con who busts out of pokey - but has to murder his chearing wife before fleeing the country. Great jailbreak at the beginning, and Reed causing all sorts of savage mayhem for rest of film. DO NOT WATCH PAST 3:37 ON THIS CLIP! MASSIVE SPOILERS!
http://uk.youtube.com/watch?v=jt6NDWtHz9YOh, and as Delvesy says, check out "Brighton Rock." Dated but brilliant perf by Attenborough. One of Scorcese's fave films, apparently.
http://uk.youtube.com/watch?v=xOfRPeqBEQ8Avoid, at all costs, "The Krays" - which is utter krap. (It stars the tossers from Spandau Ballet, so you can guess how shite it would be.)