I think the difficulty of this approach is the huge imbalance in population north and south of London. While the Portsmouth/Southampton/Bournemouth area is arou

I think the difficulty of this approach is the huge imbalance in population north and south of London. While the Portsmouth/Southampton/Bournemouth area is around 2M, the Birmingham area is about 5M and HS2 will likely also end up serving trains from Manchester and Liverpool (combined 7M) and Glasgow (2M). There just aren't enough places south of London for all those trains to go.