Personally i found there isn't enough value offerings available. It's always been advertised as faster speeds being the preference to advertisement now. Or at least since FFTC became more common. I've only ever known £20 packages are the lowest available, and that's ranged from ADSL 10/1Mb to 200Mb. I'd maybe drop my current speeds if something cheaper was available on a fixed line.
There's a minimum cost associated with actually providing a service in the first place, £20 today was about £14 in 2005 and nobody ever did broadband at that price.