Last time I took a plate off a car I just did the retention on line, about a fortnight before the car was due to go, the V5 turned up before the sale date & although I probably shouldn't have, I ran round in the car still with the old plates on for the fortnight. The dealer I sold to was going to put new plates on it anyway so I just unscrewed the plates as I handed over the keys.
You will struggle with WBAC as they will want the new V5 in their hands, if you do the retention that day you won't have the V5, If you do it after you've sold to them, the plate is theirs not yours.
I was told by a main dealer only last week that if I was trading in a car with a private plate to them, there was something like a 15 minute window whereby they could transfer the plate off & assign it to a new car & do the ownership transfer of the old car all in one hit. They would then be happy to await the new V5 turning up in their name.
You also need to be aware that DVLC may need to inspect the car the plate is coming off. I had it a few years back, transferred the plate off a BMW & got the retention slip, & sold the car the minute the new V5 turned up. DVLC wanted to inspect the BMW 2 weeks later fortunately the dealer sorted it but he did need to present the car at a DVLC office - the car by that time was up in Manchester!