Well you often can make it able to "process" 1m extra people: You can build overpasses, and tunnels, and taller buildings. But the cost-per-extra-person will tend to go up accordingly, to the point where you could spend an extraordinary amount attracting people out of the centre.
E.g. London's "Crossrail" / Elizabeth line cost $24 billion. Granted, it also allows some people to go through London faster, but I can't help to wonder what that money could've done if applied to attract businesses out of the centre instead. E.g. upgrading links between towns on the outskirts, upgrading town centres, and generally try to make it more attractive for businesses to be located further out.
Given the extraordinary costs it takes to do large infrastructure projects in London, I'd be very surprised if you couldn't get a higher return on investment that way, or by investing similar sums elsewhere in the UK entirely.
E.g. London's "Crossrail" / Elizabeth line cost $24 billion. Granted, it also allows some people to go through London faster, but I can't help to wonder what that money could've done if applied to attract businesses out of the centre instead. E.g. upgrading links between towns on the outskirts, upgrading town centres, and generally try to make it more attractive for businesses to be located further out.
Given the extraordinary costs it takes to do large infrastructure projects in London, I'd be very surprised if you couldn't get a higher return on investment that way, or by investing similar sums elsewhere in the UK entirely.