Using your British car on the continent
Gone are the days of painting your headlights yellow and putting sticky tape on them in order to use it on a continental motoring trip. These days, the only reason that people paint their headlights yellow is so that the neighbours will know that they have been to France for their holidays.
You might want to put black tape on the lights but that is only a courtesy and no longer a legal requirement.
The only things that you must do to be legal throughout the EU are to carry the correct papers together with a warning triangle a first aid kit and a fire extinguisher. You do not need all of those things in every country but it's a pretty simple set and it does for them all.
As far as insurance goes, your UK policy must, by law, cover you for at least the legal minimum in all EU countries and so, you no longer need a green card. However, unless you tell your insurance company, you may find that you don't even have theft cover let alone comprehensive cover. Also, even if your insurance company offers "unlimited green cards" they will expect to be told if you are actually keeping the vehicle outside the UK.
There are also things like bail bonds to think about in Spain. These are not required by law but, without them, you may find that you get locked up if you have a minor accident until you can come up with some cash as a surity.
Foreign police will expect to see the car registration papers, MOT test certificate and insurance as well as your driving licence. Of course, the chances are that they have no idea what they are looking at but you just might meet one who knows what an MOT looks like.
I've been stopped many times in Germany while driving a UK registered car. Not, for offences but just because in certain parts of the country, the police are in the habit of setting up check points and stopping all cars.
Normally what happens is that one officer stops you and looks in through your passenger window. He then shouts to his colleagues that there is nobody driving this one so they agree to let you go on your way. Sometimes, they argue amongst themselves for a while about who can speak English and then pick on one (usually the youngest) and he comes up the drivers window and asks if you speak German. When this has happened, as long as I have been able to produce a piece of paper on which is written the car registration number, I have been waved by.