It would be unusual, Mikey, if you were paying less with BT than with Plusnet because Plusnet is the budget arm of (and is wholly owned by) BT.
I've read loads of articles and reviews about ISPs, both online and in computer magazines and, if I was ever to consider switching to a 'budget' ISP, Plusnet would definitely be the one I'd go for. (My current ISP, Claranet, mainly provides services to businesses - including major companies such as BAe Systems - and charges 'business' prices. However that means that their phones are always answered on the first ring, any rare outage of service is normally fixed within minutes, rather than days, and I get twice the download speed of other properties on my street, with no lowering of speed at peak times).