I have a friend who is held in incredibly high regard and has a very impressive client list and setup. I know he has travelled around to look after clients' cars. He gets lots of work in and I can personally recommend his work. He's also fully accredited and spends some of his time tutoring and teaching new detailers and detailing businesses. The downside? He's based in Chepstow but, as said, he has done mobile work. I think he even has collect / delivery services as well these days (as well as storage, etc).
Depending on what you want doing it might not be cost effective to have him visit from so far afield but I thought I'd put it out there! His website isn't great so probably best looking on his Facebook page.