1. Perform a HPI check to ensure the car is free from finance and previous accident damage
2. Ask to see a copy of the V5 (the UK registration document) and MOT cert
3. Get a sterling bank draft made out to the registered owner as listed on the V5
4. Send an image of the bank draft to the seller in advance of traveling to give them a chance to verify it with your bank
5. Travel over to UK with bank draft and pick up car.
Make sure you speak with seller a few times and satisfy yourself of his / her authenticity. Anything at all suspicious, Walk away.
For a car of value < 5000, I would just pay cash. Make sure you're paying the money to the person who owns the car