Hi Gordon,
Just been on the North Midlands VTTA webpage and near the top of the page header they are quite specific that you do not have to be a member of a local club to enter VTTA timetrials. But, like yourself I can think of many reasons why it is advantageous to be a member of a local club and for that matter the VTTA as well. My reasoning/argument would apply regardless as to whether or not someone travelled around the country as part of their job.
John Ex