A chap called "Blas" at the local Ford dealer (Jerez Ford) will sell you a car and give you a guranteed buyback figure.
You can drive all the way down from the UK - I did it and the Welfare Officer at Jerez (Ann) has a handout that I wrote for her on doing so.
You can run your car on UK insurance for the time you are out there - not strictly legit but good enough. Go to
www.surinenglish.com and search the classified section for insurance (I used a firm called Seviour - very helpful, run by an English chap and his daughter).
If you run your UK car out there you will either have to drive it back to the UK for an MOT or get a local ITV test done (as long as the insurance company are happy - ask your agent).
From Malaga there are basically 2 ways. Easiest navigation is along the coast road to Algeciras and then north passing by (but not through) Cadiz.
or the quickest is to go north out of Malaga to Antequera and then west via Villa Martin and Arcos to Jerez. Once you reach Jerez, just follow the signs to the airport.
If you decide to nightstop in Spain en-route, remember that any "Hotel" with lots of neon lighting and the word "Club" prominently displayed is probably a brothel (there is one at the airport, too!).
Petrol in Spain is cheap (about 55p/l) and diesel even cheaper. Gribraltar is cheaper still - especially if you spend lots of money in Safeway and get the discount.