Great question. Registering a car in Spain is expensive - at least 1500 euros and there may be some import tax to pay as well. Then there is Spanish car insurance on top which is also not cheap.
Used cars in Spain are also expensive when compared to what you can buy in the UK and usually ex rental and knackered.
I reckon the best way is to keep your car on UK plates and insure, MOT and tax it in the UK once a year which means bringing it back to the UK once a year to do it. Not such a problem if you are visiting regularly and the winter months provide cheaper ferry crossings.
The only downside I can see is the right hand drive issue which is only a problem at toll booths when you are on your own and the gate at CDA of course. Also you would have to watch what you say to your UK based Insurers who may have an issue and try and load your premium.