Spot on David. That is what is supposed to happen with Plusnet, except for one thing.
At the point where the user speaks to them he should not give the 14 days notice. Just request an entry on his account of the kind you suggest. There was miscommunication in this case, probably involving a less than competent CS person and perhaps careless wording by the OP. Sorted by the more competent one he later spoke to. The migration notification from the gaining ISP to the losing constitutes the required notice, this being specified in Ofcom GC 22.
Slightly different circumstances when I left Plusnet under GPL migration - I wanted to retain my referral credits. I organised this with Plusnet in just that way and notes to that effect were placed on my account. All went smoothly, the only complication being that somewhere in the process I had to give them a new direct debit authority as it involved them setting up a new (email only but with unchanged username and password) account for me to receive and pay out the credits.
The suggestion here by others of placing the migration order first would not work, assuming an online order with an efficient gaining ISP. The gaining computer system in both my phone line (BT to Pulse8) and FTTC (Plusnet to AAISP) caused me to receive automated emails from the losing providers in far less time than it would have taken to get through to Plusnet on the phone.