I bought a Range Rover in the late 90s that had M2 JBM on it. When I sold the car, the new owner didn't want the plate so I kept it on retention. The certificate was about to expire (used to be 1 year) and so being a Scotsman I realised it would be cheaper to just put it on my car. Been on them ever since. A few years later I bought M22 JBM and M222 JBM for £250 each which is the lowest price direct from the DVLA. So the wife's car is M22 and my motorbike is M222.
Then, back in 2011, I knew that my name plate would come up and that I would kick myself if I didn't buy it. It cost me the princely sum of £499 and i've since turned down £3k for it. It's on my daily driver 5 series and it AT11OLS. I haven't bothered with the black bolt between the 11, if you know my name you can see it. If you don't, it just looks like an 11 plate. I like that stealthy thing.
I've had other cars that came with plates but have either given them away or left them on the car when sold.
I did also buy MC10UPE when it came out for £399 and sold it to BMW UK for their press M1 Coupe (the nice orange one) for £2k.