If your sole criteria is price, you’ll not beat PlusNet, they’re the cheapest in the country for standalone ADSL currently.
There’s cheaper to be had if you factor in other things. For instance, if you get the base package and already have a mobile with them, O2 is like £2/month or something silly. The basic Sky ADSL is free so long as you migrate your land line to them.
BE Broadband everytime. Maybe not the cheapest (but still cheap), and afaik they dont do line rental, but it is a very good service. Great customer care and they dont throttle or restrict your bandwidth.
Be is limited in availability, if its at your exchange I’d go for it.
Im paying £24pm for Sky line, calls and BB and its fine. Make sure their LLU product is available first, otherwise you’ll be on a std BT sourced product, which does get throttled at peak times. Samknows
Will tell you all you need to know.