All vehicles in Europe must have a registration document (log book). If it does not, walk away from it as there is probably something wrong with it, no matter what excuse the owner comes up with.
Also check the VIN number on the chassis. It should be easy to find as this forms part of the cars annual inspection. The owner should know where it is located.
If you cannot find it or it appears to have been tampered with, damaged or altered, forget the car, there are plenty more good ones out there.
If you buy the car, the owner should be able to arrange an export document and temporary registration plates for the drive home.
To register a left hand drive car in the United Kingdom, the DVLA needs a certificate of conformity from the manufacturer (normally a formality), as the vehicle has already been registered within the EU, but it is a red tape rule that needs to be conformed to. It can cost up to £100 for certain cars, so check it out first.
In addition the vehicle will need a UK MOT and do not forget that you will need to change the LHD headlamps to UK RHD ones. Then you need to register the vehicle with the DVLA (check out their website). You will also have to insure the car, pay a fee for registration and 6 or 12 months road tax.