Hi Sabine,
I originally brought my car over from the UK over 2 years ago, my insurance company allowed me 60 days at any one time out of the UK.
I then changed it onto Spanish plates at a cost of around 700 euros.
The only problem is that if you want to sell it, you will only get Brits buying it. I actually advertised it on here some time ago for a very good price, fully legal, with the Spanish equivalent of Mot, logbook etc.
I didn't even get one enquiry, and it was only 4 years old with a low milage.

As you ask, yes i think most people still driving on English plates are uninsured, therefore without road tax and MOT's as well, because it is impossible to have all of that unless you take the car back every year and pay your English road tax..................i dont think that happens too often do you?

I now have another English car here as well, which i bought when i went back to the UK at Christmas, i have just paid my British road tax again so that my insurance is still valid.
I have decided that i wont change it to Spanish plates because it is too expensive and then you cant really sell it on anyway.

I had my car in the Subaru dealership in Tarragona for a repair recently, and enquired how much trade in i would get against a new one.
Apparently not very much at all, because basically they cant sell them on either.
So unless i take it back to UK and sell it, i will have two cars that are useless to anyone except me, because i am not prepared to virtually give them away.

Hope this helps.
Jools