It would be interesting to know why you are unhappy with Plusnet....I am with John Lewis Phone and Broadband and the service is provided by Plusnet and I find it excellent.
Whichever one you think of changing to, Google reviews of ISPs and you'll find they they are absolutely rubbish. I'm with TalkTalk, who get awful reviews, and I'm very happy with the service and price. Plusnet are one of the "good" ones, from what I've heard, so think very carefully what the problem is that you have with them and be aware that a different ISP may be no better.