Thea is still a lot of work. Either way, you can’t run all London, Birmingham, and Liverpool services to Leeds & Manchester.
I don't think its anywhere near comparable to any of the other projects being proposed in this thread, which would likely require massive remodelling in addition to substantial new construction.
You only need platform extensions at Leeds, York, Newcastle and possibly Hull and Scarborough.
Scarborough and Hull look workable, at least for getting a long enough siding to get the train clear of the other platform approaches, without going outside the railway boundary.
York looks the same, if you use the longest platforms and remodel the south approach, but more involved given how much stuff is packed in around York.
I can't comment at all on Leeds and Newcastle due to the complexity of the station and the annoying tendency forr it all to be under giant roofs.
Can anyone authoritatively comment on the practicalities of providing 400m platforms at these stations?
EDIT:
Simplistically, at Newcastle, you could probably sacrifice the provincial siding (does anything use it regularly) and the up slow track (that bypasses the platforms) and then move the pointwork around to get enough space by allowing the tail end of trains to stand on the viaduct at the west end of the station. Access to the driving cab would be problematic though, so the train might not be able to reverse.