Just a few observations:
There aren't many XC service to/from Penzance
There are three levels of service north from Bristol, express to Nottingham, stopper to Birmingham and fast to Newcastle. How often would these run?
Similarly, there are three trains from Brimingham
Why Nottingham? Would the current Cardiff - Notingham trains be discontinued?
the overlap keeps long-distance connections running, and speeds them up, while incrasing point to point frequency between nearer centres.
i.e. Birmingham to Bristol
overall: 3tph
slow: 1tph (semi, maybe 2 with a real 'all shacks' via Worcester)
fast: 2tph (Cheltenham & Parkway only, through to Newcastle & Nottingham)
2nd example-Leeds to Newcastle
overall: 3tph
slow: 1tph-Terminates-TPE (Garforth, York, Thirsk, Northallerton, Darlington, Durham, Chester le Street)
semi fast: 1tph-Terminates-XC (York, Northallerton, Darlington, Durham)
fast: 1tph-Edinburgh-XC (York, Darlington)
this could be augmented (as has been said) with wiring through to Leeds from York, allowing EC services north of york from Leeds, so maybe giving an hourly service York-Aberdeen with alternation of Leeds & London as destinations. This in addition to a Newcastle-Aberdeen semi fast and a Newcastle-Edinburgh stopper.
As for Nottingham-Cardiff, I'd imagine this would continue to run, and possibly be extended to Lincoln & Cleethorpes. Giving 2tph Nottingham-Birmingham, and allowing the Scotland services to skip Burton & Tamworth.