The thing that kills me with these stories (and if you read the comments there's plenty more) is that customers _remember_. I'll never use Videotron again after the crap they put me through when I cancelled. I'd love to blame monopolies (sure, pick the other guys, but you'll be back in a few years when they piss you off) but the other big offender here is gym memberships, and there's no shortage of choice there. I th…
And just like with ISPs, all of them are equally scummy, so it is indeed a monopoly/oligopoly of scumminess.
UK has fairly good consumer protection laws, in fact if you express your intent to cancel, they refuse for an invalid reason or ignore it and you cancel the payment and let the "debt" accumulate there isn't anything else they can do besides threatening letters, a black mark on your credit report (which can be removed with enough effort) or debt collectors (who are actually reasonable and will drop the matter once you explain the situation).
They could go to court, but they won't because 1) they know they are in the wrong and 2) it's not worth their time anyway. The mark on the credit report is pretty much the only leverage they have, and unfortunately it works due to most people either being afraid of it or actually relying on easy access to credit.