Doesn't matter what the ASA and OFCOM think
As the title says, it doesn't matter what the ASA and OFCOM think, what ISPs are doing has no legal standing. On the contrary, I and I know others have successfully taken their ISP to the small claims court when we've been bottlenecked for breaching unpublished barriers.
Demon Internet imposed a a bandwidth cap a few months ago but refused to publicly state what it was. When I got a letter saying I'd be limited to 128kbps speeds for a full month for imposing a 60gb cap I put in a small claims court challenge online. I claimed for 1 months subscription, sign-up costs to a new ISP, 1 months loss of online gaming subscriptions (XBox live and World of Warcraft), loss of business due to the fact I was unable to realistically perform my web design business on the connection (they say 128kbps, but it's more like dialup 4k/s downloads etc.). Demon settled for a few hundred quid - they know they don't have a case.
I suggest more and more people do this, perhaps when it starts costing the ISPs more to rip us off than they gain from ripping us off they'll start listening.
All that said however, don't be fooled by Plus.Net's supposed transparency either, it is Plus.Net I have moved to since dropping Demon and although they claim (or claimed on the now obsolete package I signed up to) that bandwidth limitations were between 4pm and midnight, they were in fact up until 2am, because P2P was still restricted further between Midnight and 2am.
Essentially, there isn't an ISP out there at the moment that hasn't jumped on the screw the customer, increase the profits bandwagon. It's sad that UK internet access has actually deteriorated over the past few years, we went from metred dialup to unmetred dialup, to unmetred broadband, to capped broadband and some ISPs are rapidly heading towards metred/capped broadband.
There's something very wrong when everyone could only get 512kbps - 2mbps for a few years, completely uncapped, yet suddenly we've all moved to 8mbps and the bandwidth caps are so low it means we can only download 1/20th what we could previously in a month when we were all limited to around 512kbps - this suggests that ISP bandwidth capacity has actually gone down, despite speed going up. The worst part is that some ISPs, like Plus.Net are even throttling us on the speed front too, so not only can't we download as much, we can't even download it as fast as we used to!