They are probably asking the wrong people.
Are you with Waitrose phone and broadband now? In which case, Waitrose are John Lewis and the underlying line and broadband is with Plusnet. Who incidentally are owned are owned by BT.
The number is probably owned by Plusnet and whoever is being spoken to at Waitrose to release it simply doesn't understand the system. They only know how to pass on an order or cancellation to possibly John Lewis or maybe Plusnet direct. They won't know how migrations work.
How you should handle this is another question, but it should be being handled by whoever you have ordered the FTTP from. Which ISP is that? BT Consumer (Retail) by the sound of it. Whatever you do for the minute don't tell Openreach they can just dump your number.
You may need to go to the top of BT with a "High level complaint". It is possible to email them and get heavyweight control from that team who report to the Group CEO. (You actually email the CEO).
If I read your post correctly, yes at some point your existing copper line will be connected to that hybrid drop cable.
My broadband basic info/help site - www.robertos.me.uk.
Domains, site and mail hosting - Tsohost
Connection - Three 4G, tbb tests 35-45Mpbs down, 9-15 up.
If you never think of anything off the wall, you'll never think of anything original.