Basically you get what you pay for...cheap ADSL deals are often unreliable, the service is capped (limit to how much you download) and you are locked into a 12 or 18 month contract. Compare the market here:
http://www.thinkbroadband.com/search/package/basic.html
The best (overall) DSL providers in the UK are Zen, Nildram, Vispa, EntaNet, IDnet, KeConnect and NewNet
http://www.thinkbroadband.com/search/package/basic.html
The best (overall) DSL providers in the UK are Zen, Nildram, Vispa, EntaNet, IDnet, KeConnect and NewNet