Sticking ONLY to Kellsmac's question.....
May be the vehicle is the wrong choice and that one with a small cherry picker on the back would be better.
However, if that is not reasonably practicable and we haven't got enough information about some variables to make an appropriate judgement, then....
HSE guidance says that a ladder may be suitable workplace for jobs of short duration and has been know to define that as tasks taking up to 30 minutes, though many would consider that if the job takes that long at a single location, a ladder is probably NOT the right way to do the job.
But Kellsmac says that this job will only take 2 minutes (which might easily be an underestimate, particularly if you include the time to set up the ladder and tie in to a column).
Fall arrest won't work if the fall distance is only 6 feet (about 1.8m), but the use of a harness and very short lanyard as work-positioning equipment might. The problem is that the ladder is probably not designed for such an attachment, and is even more unlikely to have documentation saying that any part is suitable for the attachment.
So, you have the three points of contact and in HSE's eyes this is probably OK, and the pole has the advantage that the sign is not dependent on an A frame being suitably ballasted so that it doesn't overturn.
Of course, the bigger risk is having a truck parked at the side of a live road (assuming that there doesn't just happen to be a parking place alongside the road!).
So, I suggest a review of the whole system of work, but with everything starting with compliance with "Chapter 8". Then you consider what tasks have to be done from or near the vehicle and what the most practical options for a relatively safe procedure are.
So, as example, people can and do put out cones on live roads without even exiting the vehicle. Hence, similarly if the putting up (and removal of signs) attached to poles is a regulat occurrence, why not put a small MEWP on the vehicle?