Yes, what is said is as good as right. If you weren't a Spanish resident it is theoretically possible for you to leave a UK reg car in Spain if it was keep off the road for 6 months a year. But them you have to prove that.
There are German Motorhomes that appear around here every summer, indeed there are German Motorhomes I see here all the year round. The difference is that they tour all over Spain I guess, whereas with a car it would be seen by the guardia a lot. I dont know how they get around the "MOT" situation
Once you are a resident then you have to get it matriculated or risk it being "caught"
Until about a year ago there were quite a few cars with UK plates on the roads around here, many of them LHD, a clear giveaway. I've been told of people who have been driving for years in UK registered vehicles.
But I've noticed just lately that there are far fewer on the roads. There are still a handful of 'regulars' I see almost every day I go into Estepona , mainly posh expensive vehicles...big new Mercs, Porsche, even the odd Bentley or two.
Whether this means that the owners of RHD cars have sold up and bought Spanish, gone back to the UK or what, I don't know. But it is an interesting change.
We've certainly seen an increase in the number of police road blocks, both Trafico and Nacional.