Hi we drove down to Spain originally though France but this expensive on tolls the best way seems to be the ferry from the South west of England to the North west of Spain and then drive down from there last time there were no tolls. It takes about 8 hours to drive down.
We also bought a left hand drive car in UK and then registered it in Spain this involves have it ITV'ed which is the Spanish MOT and usually includes the changing of headlights cost is about 90€ plus any costs for new lights etc. You then have to go to traffico and apply for new number plates which I think cost 70€ and another 25€ to have them made up.
The easy way to do this is to employ someone to help with the translating etc if you what someone I can put you in touch with someone who can help.
The other thing to do is to go to Alicante to the British Consulate and get a document from them to say that you brought the car into the country has your own personnel property so you do not get changed import duty. It does mean that you cannot sell the car for 12 months or you would have to pay the duty.
If you buy a left hand drive car in the UK and it is fairly new you may not have to have the head lights changed (we didn't has the lights were centred not off to oneside so did not need changing.
Second hand cars are expensive in Spain but If you look around you can still get one at a reasonable price.
Hope this helps and if you want any more help pl,ease send me a private message