They aren't allowed to under the new (no MAC) system.
They should have notified you that they are aware you are leaving, and told you of any liabilities to them you might incur such as cancellation charges or minimum term in force that would need paying off. Is there an email lying around anywhere, perhaps in a Plusnet mailbox you ignore?
You can however get in touch with the Customer Options Team (normally called Retentions) and ask if they can do a deal to keep you. Because you instigate the conversation they are allowed to make an offer or offers. It just must not start from their end. You just asking normal CS if they know you are going doesn't start such a discussion.
Kindness isn't going to cure the world of all its awfulness but it's a good place to begin. Daisy Ridley.
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