There are still a few around where I live, but they are on a street which has very run down shops, which mostly consist of a mini-market shop every other one, kebab shops and cafes that the local Muslim men like to congregate at after prayer.
As such, these internet Cafes look really dodgy and given that they are still going, I would think they probably serve a lot of the immigrants who come into London from abroad and likely don't have internet or a computer. I'm sure that they are probably also where most of the UK based Nigerian scammers operate, as most of them also advertise money transfers at the door.
I'm sure for anyone wanting to setup an internet cafe though that this isn't the market you want. As others have said, that ship sailed ages ago. Back to square one, make a decent cafe and have free internet and services as a bonus.