Unfortunately Manchester and Birmingham already require a change. I always thought the number of journeys where Euston-Paddington transfers are removed with a change at OOC was a massive benefit of HS2 that wasn’t shouted about enough - or does everyone expect me just fly from Scotland to the south west?
It wouldn't be any faster going from the SW - and by that I mean west of Taunton so it's not taking some near-Bristol advantage - to Scotland via OOC than existing XC. The only section that'd have done anything at all for the south-west is north of Birmingham. it's defintely not faster going to Birmingham. As I've said before, you can fly to Glasgow from Exeter, turn the aircraft around properply, fly back, turn it around again properly & fly back to Glasgow and the train still won't have got there, so if speed matters you're always going to fly.
By "turn around properly" I measn disembark everyone, refresh all the consumables that need to be refreshed, and then embark a new load, rather than just touch & go & head off.