The paths to Marylebone are full in the peaks too, so that's a non-starter....
Signalled to the hilt with the shortest practical sections, all occupied by trains of the maximum length the line can support, and with the very best of performance curves equal to that of an electric unit, I expect. Ah well ...
I'm somewhat playing Devil's advocate here, but Oxford to London via High Wycombe is a potential diversion / relief route - into London Paddington as well as into Marylebone (there is some remaining capacity from the Chiltern line into Paddington, isn't there
) - and at the time those diversions or relief is needed, trains might continue west from Oxford
You'll note that my comments / this option does not rely on a re-opening from Kingham to Cheltenham Spa to make it useful - however, it would provide for a route shortening / alternative to Cheltenham, Gloucester and perhaps South Wales if that re-opening was implemented ... which strikes me (I admit) as improbable in the foreseeable future.
