I think it best to appoint an Agent to act for you if you are not accustomed to doing this. I bought a 1951 MG TD from a Dealer in California who was used to dealing with exports to the UK and he was able to suggest an Agent to me.
The Agent will "hold your hand" through the whole process and point out any potential problems before hand so that you can deal with them before you are faced with them at the last moment. In my case, for instance, the Agent said that whilst my car looked like an MG and had MG written all over it by way of badges and so on, Customs would not believe that it was an MG unless I had a letter from one of the UK MG Car Clubs certifying the age and that it was a genuine MG, which could be presented to them as the car came in to the UK!
Depending on what you buy, you could be faced with VAT and also Import Duty - together with all handling fees and so on. When you have decided what you want to buy, you can telephone Customs and Revenue and they will let you know on the phone what rate of VAT you may be faced with on the age of the car that you are buying..........
There is more to it all than this brief note - but good luck with it!
Tony