The people moving into your old flat would not have been able to order broadband and possibly a line if there was an open account at the premises.
Suspect that is why your account has been closed.
The new occupants now own the landline & are entitled to do what they want with it. You have no say on whether they switch to a new ISP. As a result your BB there has been ceased. You should be paying less in Early Termination Fees than had you seen out the contract.
You effectively did cancel your BT contract early by vacating the flat where the service was laid on. You left the flat knowing full well that the next occupants were quite likely to want to take over the line. It's your responsibility. You would have "released" the flat with vacant possession, which means you have no title at all to anything there.
You should have returned the BT kit to them when you left.
If you move out of a property without cancelling or moving the phone line to your new address ( thinking you can just continue to pay the monthly fee until the end of the minimum term) that is only going to work if the new occupant doesn't want phone/broadband service themselves from the likes of BT , Sky , TT etc, because as you have found , if the people moving in want service and apply to a service provider , simply by applying generates a letter from your provider to the address you have left, stating someone has applied to takeover the line, and if that's not what you want, call your provider to cancel it, otherwise in 10 days the line can be taken over, obviously as you have left the address, that letter goes unanswered, your old provider not getting a response to that letter lets the line go, and 'ceases' your account, so any charges etc, get calculated and a final bill generated, and the new occupant gets service (after the 10 day wait)
It could have been worse, if you left a working line and broadband in the flat, and the new occupant realised 'free phone and broadband' they could have and used/abused it, knowing that it would be you as the bill payer that would be responsible for that usage and have to foot the bill.
You should have realised the new occupant would probably want service, it wouldn't be fair for that person to be told tough luck , you cannot have service until you ( the previous occupant) called and cancelled your service.