I expect the majority of people on here would recommend you go with Plusnet, although there are a few trolls who will pop up to tell you not to touch them with a bargepole. For me one of their main advantages over BT is the UK-based support (and yes, I know BT owns Plusnet, but it's not directly relevant here), although response times through the official channels can be somewhat slow.
You seem to have done the sums, although I haven't checked the figures. Have you considered the line rental issue and do the figures assume you will be moving your line rental as well? Also, Plusnet are extremely likely to be launching their own TV offering soon, which may change the calculations.
Ask away if you have any specific questions.
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