The valid routes from Nottingham Group (the appropriate routing point [along with Long Eaton and Loughborough, but you have to pass through Nottingham to get there, so they are discarded]) to London Group (of which Old Street is a member) are on maps KS, NM and PN+KP, so here are the permitted routes, including the maps used:
- [KS] Nottingham Group - Grantham - Peterborough - Hitchin - Stevenage (via Hertford North) - Finsbury Park (via Highbury and Islington - Essex Road) - London Group
- [KS] Nottingham Group - Grantham - Peterborough - Cambridge - Hitchin - Stevenage (via Hertford North) - Finsbury Park (via Highbury and Islington - Essex Road) - London Group
- [NM] Nottingham Group (via Derby) - Loughborough - Leicester - Kettering - Bedford - Luton - West Hampstead Thameslink - London Group
- [NM] Nottingham Group (via Derby) - Loughborough - Melton Mowbray - Leicester/Oakham - Kettering - Bedford - Luton - West Hampstead Thameslink - London Group
- [PN+KP] Nottingham Group - Loughborough (via Leicester) - Melton Mowbray - Oakham - Peterborough - Hitchin - Stevenage (via Hertford North) - Finsbury Park (via Highbury and Islington - Essex Road) - London Group
- [PN+KP] Nottingham Group - Loughborough (via Leicester) - Melton Mowbray - Oakham - Peterborough - Ely - Cambridge - Hitchin - Stevenage (via Hertford North) - Finsbury Park (via Highbury and Islington - Essex Road) - London Group
This allows you to go to Nottingham and then direct to London St Pancras International (and I think then you would have to make your own way to Old Street as interavailability only applies if you come into KGX even if your ticket is valid to both) or via Grantham or (Leicester and) Melton Mowbray to Peterborough and down the ECML or via Ely or Cambridge (though not into London Liverpool Street) to Stevenage and then via Potters Bar or Hertford North to Finsbury Park, where you can either go to London Kings Cross and use the interavailability on the Northern line to OLD or down the Northern City line from there straight to Old Street. Of course if you do this your train doesn't need to stop at all the Routing Points/Routing Groups, just pass through them, as it is one ticket not split at them.