The original MSL London Extension project (Act of 1893) was for lines Annesley-Quainton Road, and Canfield Place (Finchley Road)-Marylebone, also a couple of connections, with running powers over the Met from Quainton Road to Canfield Place. During the planning/construction phase, the Met agreed to build two new tracks for MSL use (exclusively) from Harrow to Canfield Place; and a new line was authorised from Woodford to a link with the GWR at Banbury, with a connection to the Met between Edgware Road and Baker Street being dropped. And the MSL dropping proposals that would conflict with the GW, including reaching Birmingham via Stratford (over the E&W Jcn, later SMJ, and North Warwickshire Lines), and - if it hadn't fallen by the wayside first - a connection to London & South Wales Railway.
The GW&GC proposals (culminating in Act of 1899) followed later, with new GC lines Neasden-Northolt and Ashenden-Grendon Underwood; and a joint line between them (and other GW lines).
Once the ensuing (or continuing) row with the Met had quietened down, and it been agreed that co-operation was better, the Met&GC Joint was established, taking over the Met lines from Harrow to the north, and the GC took over (long lease) 'its' Harrow- Canfield Place tracks.
One wonders might have happened had Watkin acquired a directorship of the GWR in the 1880s, and thought about that as a way into London.