It's about £80 to get a no plate held on retention per yr (you pay DVLA for the "privilege" of not reassigning it). The no plate companies that sell them generally will offer you half from my experience. I've never sold one but offered a few for sale just out of curiosity. I had J2AOS or something similar, offered me £400 and advertised it at £800, they cover all fees, organise you a new plate (an unassigned plate from the same year). All you need to do is offer it for sale and if someone wants it, they contact the company, the company contacts you and sorts it all out. So, in theory you could advertise it as is, then if someone wants it, you can go get your MOT and tax, swap the plates, and it's cost you the price of tax and mot. Sometimes takes forever though, my brother had A141MAC and advertised it in about 2001, it was about 2007 before anyone contacted him (he'd sold the car 6 months after advertising the plate).
So, yeah, you may be lucky but don't expect it to be a quick sale, depends on the perceived value of it.