With the larger lorries in use today I think 3 could probably do the job of a pair of narrowboats. The lorries would each need a driver and could do London to Birmingham as an example in about half a day, so about 1.5 working days worth of labour. London to Birmingham by canal normally takes about six days and the pair of boats would be worked by a crew of two, so about twelve working days worth of labour.

'Fly' boats working through the night could do the trip in about 52 hours, but then you'd need extra crew to work the extra hours. Given that there are about 100 locks to work through between London and Birmingham this timing was pretty good.

You can walk from London to Birmingham along the towpath, as I have done many years ago when I had better legs, in about the same time as it takes by normal boats.

About the only way to get the costs down would be much larger craft, which would need much bigger canals.