When we had our line here we had a billing dispute with BT which resulted in their normal response - simply cut the line off. We got a reminder, then a cease notice etc.
It wasn't possible to resolve the issue, as a resolution requires the other party to address the issue and reply to you, so the cease went ahead, the line stopped for calls, then we got an email from Zen that the ADSL was to be ceased.
Since I wasn't going to be pushed around I told them fine, go ahead, we'll cease and get a new number from someone else,
Zen then refunded the unused portion we'd prepaid (very nice of them), we bought a 3G modem, found it faster than the ADSL was anyway, and haven't used the line in over 3 years.
So I suspect you have a "cease notice" coming from Plusnet soon.
Odd that BT will take you back as a customer, when clearly, they didn't want to keep you - anyone might think it was just a way of making extra cash out of people.