Ha, my mum traced my dad's family. Went back 350 years and, until my grandad, they all had the same job (carman) and lived within the same mile or so in Bermondsey!! 350 years! Unambitious bunch.
You're completely right. Essex and Kent have a lot of crime guys, but the structure in the UK means they mostly act on their own. And the older they get the less active they are. In the pubs in south east London / north west kent (where i grew up) there are countless guys described as 'gangsters' and lots of people walking on eggshells around them. But a five min chat or a bit of research usually proves they're nobodies.
I know very little about Bangladeshi/Pakistani gangs other than I know the local protection rackets in Bethnal Green actually work. Some drunk guy started causing trouble in the newsagent my girlfriend lived above a few years back. According to her, the newsagents kicked this guy out the shop but he came back 5 mins later. Shortly after three Transit vans pulled up outside the shop and out the back jumped dozens of young kids all carrying planks of wood and baseball bats and things. They dragged the guy out the shop and chased him off down the street. The newsagents were definitely not properly connected to anything, it was run by a sweet old Bangladeshi couple who'd been there for years and their three shy, geeky sons. Rumour has it they just put in a call to whoever they pay protection to when the guy showed up again and, well, they were protected...
British 'Firms' ( it's the term mostly associated with British organized crime and sort of a local equivalent for 'mafia' ) usually operate in small cells. Sometimes as small as having 5 people in a group. Sometimes these groups cooperate with each other and sometimes they fall out with one another. There's no large structure as the Italian, Turkish, Kurdish, Albanian,...groups have. But on the other hand there are a shitload of those smaller groups throughout the country.
I don't think the Bangladeshi gangs are anything close to the Turks, Russians,...or even Pakistanis and British for that matter. But small protection rackets are, as in almost any big ethnic community, of course a given
