Gill Wise does clinics at Cobham Manor. Heard she is very good. Have been judged by her but not had a lesson with her myself. Thinking about it though.
Sue McMahon teaches my friend in Littlebourne. She's a list 1 judge I believe. She's given my friend some lessons on Mickey, very knowledgeable and made a big difference to Mickey.
Other than that I'd say Marion Kendall but I am not sure if she travels, last heard of her in the Dartford area. She's a list 2a judge/trainer I believe.
Ali Ferry (based in Chilham) may be an option too.
Ali rides my pony and is a very good friend. I can highly recommend but she rarely gets away from Chilham so you would have to travel to her (I think).