Again, have a chat with FTE, as it's a common question they're used to answering.
I hear there's a local bloke who will sell you a car at the start of your course for maybe £1000, and will guarantee to buy it back off you at the end for £500.
There are a couple of issues regarding UK registered cars after about 6 months out there, but nothing more than sorting a bit of paperwork. It seems the best bet is to get a banger out there and check with FTE and the local Jerez Police to see what paperwork you might need.