I can highly recommend Ben Mayes - he is based near Horsham, and is a fantastic vet. I attended many vettings with him, and he is very thorough and also knows a lot of people so can help find out history of the horses etc that the buyer might not be aware of.
I have used Steve Brooks for 10 years and he is very good. He did a 5* vetting on my mare a few years ago and never had any problems! He is very close to Gatwick.
I have also used Shotter & Byers for an emergency once and again they were brilliant.
Another vote for Alastair MacVicar of Anvil Equine, he has just done our vetting for us, I was there he was very thorough and very good. I am told he was an ex FEI vet but did not clarify this with him.
Have his card in my purse Tel 01403 731213, my own vet used to work for Arundel and told me to make sure I asked for a partner to do the vetting there if I chose them.
Good luck, are we allowed to know where you are looking?