OP – if I’m reading your post right You can do it all online now….
We were getting a new to us car. This is what we did….
We were due to collect the new car on the Saturday so We put the cherished plate on retention online on the Monday at the start of that week.
This generated an emailed retention certificate advising us to put the original plates back on the car.
It also generated a new V5 in the post from the dvla which we received on the Wednesday.
So when we took the car in to pt ex it it had the original plates on and the correct V5.
We collected the new to us car on the Saturday. We Received the v5 for that vehicle about a week later. Went online and assigned the plate to the car. this meant we could immediately put the cherished plates on it
Not sure if you follow that (or if it helps)!
Check it out online but I think you’d be best doing what we did and putting it on retention online – to do a transfer will take a lot longer