Buy an old 50cc moped, transfer the number to it, put it on SORN and stick it in the shed 'til you decide what you want to do! I believe (not sure, but it's what I'm told) that nowadays retention has to be renewed every year, at a silly fee! I used to have a personal plate and that's why I let it go with the car.
'i still never understand why anyone wants to bring a UK car to Spain as a RHD car, much better, easier and safer to have a Spanish car in Spain' - I appreciate what you say, Tadd, but my main reason for doing it was the saving in cost.
For example, 5 years ago I bought a 7 year-old car for £500 on ebay, drove it to Spain, total cost (ferry, fuel etc) about £250, spent two weeks of my retirement putting it through the system (using S/H parts as necessary) cost under 200 euros. All in all, a shade over 1,000 euros total, whereas the equivalent 'local' vehicle at the time was around 2,000+. It's current insurance valuation - their figure, not mine - is 1.500 euros. I have to confess, after being a professional driver for over 45 years, and having driven RHD lorries on the continent, a RHD car does not bother me, and neither my wife or daughter are phased by it.
Steve