Liverpool is a ground zero for 'crime firms'. There are so many small groups of organized thugs active in the city it isn't even funny. The thing is though, as of late there doesn't seem to be a firm with a steady power in the city. One firm may stay on top for a couple of years, but another one quickly seems ready to take their place.

Glasgow has been 'controlled' by the Daniel family for years, but apparently they have diminished their activity and a few mobs from the city's southside seem to be coming up, ready to take their place.

The London area still has some behind the scenes activity from the traditional powerful families: Adams, Arif, the Canning Town group (who are now mostly based in Essex)...though the presence of any of those groups isn't as aggressive as it once was. Especially the Adams', Brindles and Arifs have taken heavy blows in the last years.
Traditional OC in the East End is done. There are barely any white working class people living in the area now. A lot of South Asians, Caribbeans and white yuppies living there now.
South London, at least parts of it, still have a more substantial British white working class population. Lots of smaller firms operating over there (and into Kent and Essex), though I don't know if their power if of the scale of the Adams and the likes.

Newcastle, Nottingham...and assorted smaller British cities will always have their fair share of firms as well.