I guess that you were either VERY lucky 😆 or they didn’t find out 😉 – or perhaps it was a long time ago. Things are getting tighter all the time and it seems to me quite reasonable that if we use the roads we should pay road fund tax either in the country in which the car is registered or where we use it – ideally the latter.
For someone coming to live permanently in Spain, and wanting to bring their UK car with them, it is relatively inexpensive to register it here provided that it is done before or at the same time as applying for residence, as it counts as part of your personal belongings. If you leave it until after you have residence it is very costly as there are all sorts of expensive hoops to jump through, as we found to our cost when we had to register the trailer we brought from the UK and we had already got our residencia. Once it is regsitered here there are no more worries about MOT or UK tax and the tax you pay is SIGNIFICANTLY lower than in UK 8) , not to mention that the first MOT is at 4 years, then at 6 and it is not until you get beyond that that it is annual (or it might be after 8, not quite sure).