BT aren't allowed to charge you if you're out of contract as it would violate some consumer protection legislation I hope. I am currently locked into a BT contract because I stupidly accepted their Home Hub offer and have no end of problems with it. I want to go with Be because I had them for a year at Uni and had zero problems with them, top notch service, 20Mb down 1.3 Up where I was and you can get up to 2.5Mb up now if you meet certain criteria on connection quality. Also only 3 month contract with Be so you're not stuck with them if you don't like them.
Done exactly the same thing here, stuck with BT for a fair few months more. The traffic shaping on BT is just dire, it's like a dial-up in the evening, with especially bad speeds on torrents.
Have heard good things about Be, might have to go for their unlimited option as soon as I'm free from BT.
This site is quite interesting: http://www.dslzoneuk.net/isp_ratings.php