I dont know how helpful this is, but I had a chap out in Hanwell who dipped doors. (Opposite the big tile shop). I know it's a bit of a way, and you have to schlepp the doors on your car roof...sorry I'm out of the country or I'd be able to give his name. But there MUST be others. Have you tried the people in Wandsworth Bridge Road? They have two or three pine shops there, and I think they would be able either to do it or to direct you.