For new or vintage UA gear in the US I would recommend James Gangwer without a second's hesitation:
http://www.jamesgangwer.com/ (I suspect US-based members will concur)
For UK-based Techs it would be horses-for-courses IMO - not to say any individual I can think of wouldn't be versatile in their skills, but Techs sometime have their "pet topic", and although others may have the skill to fix / modify a wide range of gear, a specialist will have the connections to get parts faster / at the right price, and they won't have to spend weeks searching for the schematic.
Drop me a PM with details of specific gear and I can suggest UK-based bods that specialise.
Justin