Please see the below post from @Kilopylae to avoid further clutter in the other thread:
My guess is the boundary could only be at a major station and quite a way outside of Greater London. It's an idea with lots of potential and lots of awkward implementation options. Obviously there would also be little way to prevent splitting, except for trains which run non-stop into the area from a long way away.A national 'trains that don't go to London' Railcard would make some amount of sense to encourage modal shift. Given how much the train is just 'how you get to London' but not a realistic option for local journeys in many areas.