Simple things like carrying spare bulbs and tools needed to change them in the car, I believe its two warning triangles, along high viz vests inside the car so they can be put on prior to exiting the car if you break down.
Halfords used to do bulb kits for most makes and models of cars. I doubt if the police would check if its the right bulb kit for your model of car
Another tip for the high viz vests is to put one on prior to exiting the car if you get pulled by the police and asked to get out the car. One of their little tricks for giving instant fines.
There has been a recent police clamp down on British plated cars in Spain.
So carry a copy of your ferry crossing details or eurotunnel bill as proof of when you brought the car over inside the car with your documents. This should be enough to save you a lot of heart ache and unwanted fines.
Its also worth checking if your MOT or road tax is due to expire while your over. The tax disk being the dead give away to if a car is road legal for England.
Also carry a couple of hundred euro's in cash with you. Pay a fine on the spot and the very nice police officers will give you a discount for paying on the spot.
Watch your speeds when driving through towns and built up areas. This is where the gestapo in police uniforms
got me on my drive to Spain.
Mobile camera hidden up and some nice police officers waiting round the corner to give the fine
There are a couple of good insurers on the Costa Blanca. Linear directa and knights. I found with my own car that Knights will try to beat a Linear Directa quote. So its is worth shopping round and sometimes seeing if they can beat each other for quotes.
You will be limited to the insurers that will insure a UK plated car
If you use a Spanish insurer, most supply break down cover in the fee. So no need for AA or RAC cover.
Good luck with your trip down
Carry two or three forms of plastic for driving through France. I had two cards rejected after using each one several times filling up. So my visa card worked well for the first one or two fills then got rejected, then my master card and then onto my other visa card.
Nothing worse than being in France in the first place, let alone with a card not being rejected at a petrol station
Trust me finding that my mobile phone would not work when trying to phone the bank, because I had not set it to operate in Europe went down well. And of course you could only contact the phone contract supplier from the UK.
So its worth checking your mobile phone contract prior to leaving the UK.
Totally uneventful drive to Spain when I moved down
car insurance 
