This could go on and on etc...But it still 'boils down ' to the fact that if you have a UK reg. vehicle here in Spain without road tax you are breaking the law here, and it could affect insurance etc..The road tax is paid wherever you live in Europe and gives you the right to drive on the roads in Europe,so if you don't pay the road tax somewhere , then you are breaking the law, which then can lead to all the complications...The Spanish have to pay road tax here , I know it's not much compared with Uk, so why should anyone think that they can drive around in Spain without paying the road tax...? I know that it is probably not what a lot of people want to hear, but I am quite sure that it is correct, as it stands at the moment..Although technically wrong, but if you keep a UK vehicle here with up to date road tax etc., it is unlikely that you would have any problems..If you can make the journey once a year to Uk, and have the mot etc,you would then be reasonably safe, but still not entirely legal...
Also if you have a Uk vehicle in Spain taxed or not, and leave it behind when you leave the country, then again you are technically breaking the law.
I am not a lawyer, so I can only give advice based on my own experiences, if you are not sure, I suggest that you make an appointment with a Abogado, and get him to put into writing how he sees it...
Anyway I repeat that I am not a 'vigilante', and only offer my advice on the understanding that it might help someone..
