Good point; If intended to replace all, or even just most, of the current London termini, how big would 'London Central' have to be ? I would hazard a guess at, roughly, Absolutely Gigantic !
We do have a kind of 'super terminal' at Kings X/St Pancras, with our one international route, two main line routes, a cross-London national rail route, and multiple Underground lines, and Euston (with, one day, HS2) is not far away either. Although, perhaps, should Crossrail not have run via Euston/Kings X too ? IMHO it is a pity that the GWR went for Paddington instead of running alongside the LNWR from Old Oak and terminating adjacent to Euston. But we are where we are.