Hate these areas of insurance - same as the way commuting often doesn't cover driving to a different place occasionally for work or giving colleagues a lift, etc.
I've pondered from time to time how it applies for those who's place of work changes.... how permanent is permanent?
If you work on construction sites and don't report to the companies office, it would make sense for travel to the site to be classed as commuting, you go there do a days work and go home, obviously this doesn't apply if you get to site and find you have a situation on another site that needs sorting (that would be like reporting to your office and then being needed at a branch office, thats business travel not commuting)
But to be classed as your permanent place of work, how long do you have to be based at one site, I don't think it should matter, you are driving to one place to do your job, and back again, but I think it might... a week? month, 6 months, a few years, hell some office workers change companies with a shorter duration than some construction projects last