Like you I also drive a lot with my work, around 35,000 miles a year, and unless the road is narrow, when single file for bike and horse is preferable, I really can't see how single file is any different to side by side. Let me put it another way, what's easier to overtake, a slow moving car or a tractor towing a small machine.
For what it's worth, when we get traffic behind us, we tend to ride the horses single file, as it cuts out a lot of the verbal abuse, but I still think it's as broad as it's long... literally.
We have this issue with the bike club, if there's 20 of us and we're riding 2 up & 10 long, I reckong this must be the size of an articulated lorry, is there really any point going single file and being 20 bikes long, two lorries long!!
Also, had a blackbird fly straight into front wheel of a motorbike I was riding - poor thing.
I found a blackbird, erm, 'parked' in my brake vent in the front valance.