There are also plenty of us who have homes in both UK and Spain. When I come over for a few months I cash in my UK tax and SORN the car. This is perfectly legal as UK road tax only applies for driving in the UK. You need to check with your insurance company and of course you need to be MOT'd for the insurance to be valid.
The EU law on this is totally bonkers. I had great discussions with Lord Adonis during the last government. "You can use a car in another EU country for up to 6 months in any 12 month period before it must be re-registered." No-one could tell me if this meant a calendar year, a rolling 12 month period or how multiple trips abroad were allowed for. I even asked if MEPs drove back and forth to Brussels and was told that many do.
Always keep a valid insurance certificate and proof of a ferry crossing within the last 6 months in the car.
I would absolutely not condone colour copying your tax disk before cashing it in nor editing and reprinting the emailed ferry crossing every month or so. That would be very naughty.  ;D