The first of those trains departs at 12:46. This is the earliest service south. Surely they have forgotten to organise replacement buses?
The fundamental issue here is that the normal route from Glasgow is blocked by engineering works. Yes, there is a route via Dumfries, but the signal boxes on this line don't open until after lunchtime on a Sunday, and Avanti only have a limited number of Voyagers stabled in Polmadie, and an even more limited pool of drivers with route knowledge of the G&SW - so this isn't really a particularly useful option.
Furthermore, Avanti don't have any facilities to stable trains at Carlisle, or even at Preston (on Saturday nights), so the first southbound service is formed off an ECS from Longsight.
In turn, that can't get into Carlisle in time to form anything before the 12:46, because the WCML north of Carnforth only opens after 10am on a Sunday. In fact as late as 11:15, between Penrith and Carlisle in the Down (northbound) direction. So they will be running about as early as they possibly can, given the constraints.
Now, I agree that the standard blockades should be cut back in the event of engineering works preventing the normal arrangements in terms of stabling, through running etc. And even if nothing else, Avanti should be running earlier buses to replace trains - as they seem to be doing the following Sunday. But unfortunately that's the way that the timetable has been planned for this Sunday and it's a question of 'take it or leave it', pretty much
