Class 323s did run on Manchester Piccadilly - Birmingham New Street, I certainly remember them, as does one of my colleagues who was an Airport guard, who also signed Class 322s and had route knowledge to Euston via Northampton. There was an overnight turn on Manchester - Brum workings.
Peak time semi-fasts (think it was a 1Hxx headcode) between Stoke-on-Trent and Piccadilly lasted until the VHF timetable was introduced. I remember them on diagrams in 2006/7, so lasted into the Northern franchise.
When LM and XC was created out of the CT carve-up, DfT wanted LM to run a Manchester - Brum service, either taking over the existing paths or as a new service, but either the figures or path didn't work out, so was quietly binned.
As for Milton Keynes, we are classed as the MKSM (Milton Keynes South Midlands) growth area or officially the South East of England thanks to the EU.
Personally, I think Manchester is lucky not to have London Midland serving it, or any of the other useless sister companies/TOCs/brands it cannot manage competently.