Okay, coming at this from a novice point of view, having never (yet) ducted an mvhr system.
I would have assumed one could get (or make) an in line coupler to join two lengths of ducting together. then no need to work it out, just pull through until the roll runs out, insert coupler, start next roll, no waste.
No I know why we don't do that with UFH as in line water connections that you can never get at are a bad idea. But an in line connection in an air pipe, that in many cases will be in a loft space etc where you can get at it does not sound like a bad idea at all.
If in line couplers are not available I think I have just found a gap in the market.