I'm trying to get more specific info on this gun and cannot yet answer for certain but I suspect it is indeed less expensive than joining and regulating two individual barrels.
They must have solved the regulation problem but their literature (scanty) only states that the barrels were machined for a calculated POI.
"The Field" had an article on an Italian maker (can't remember for certain - maybe Piotti?) that also was producing an entirely machine-made top-end gun (about USD150,000) but they didn't mention that the barrels were monolithic.
If the Hesketh shoots well and has a sweet feel to it then USD15,000 isn't bad. I would love to handle one. They have one in Lancashire that you can shoot on Sundays.
Mike Yardley who is (in my opinion) a pretty good gun critic had nice things to say about the Hesketh.
I also suspect that every metal piece on the Hesketh is interchangeable with no hand fitting required. It's an extremely sophisticated manufacturer - take a look at their other website that can be accessed from the gun site.
