I think London is a bit of a different thing.
Whereas in Vienna they demolished Sudbahnhof and built the huge Haufptbahnhof which has reduced Westbahnhof to just local services.
I can't think of a place in the London area that would be a really prime site for a London Central. Now Maybe the Old Oak area is about the only place it could be done.
Vienna Hbf is big and centrally located, but yes, Vienna and London are very different beasts. It's a through station and also walkable to the main areas of the city, which is far more walkable than London. Vienna Hbf carries all international and intercity services in one station and has about the same footfall as Waterloo per year, but Waterloo is just one of 14 London termini, and has no real intercity services!
The main benefit for Vienna Hbf (other than double length platforms) is that it is a through station convenient for all the main areas, but also with viable through routes that trains can serve and only spend a few minutes occupying platforms, unlike at termini.
All our main London stations (as with Paris) are termini, but even if there was one through station, there are a few issues:
- The sheer size of it would need to be truly enormous for the footfall - dozens of long platforms on different levels.
- One central location isn't very useful for most destinations in the city - one station serving everywhere from Kensington to The City to Shoreditch would need good "Crossrail"/"RER" type connections.
- It would need to be a through station but where do you terminate the routes? Through services would need to connect up but London is not at all central to Great Britain. Vienna can serve Prague, Munich, Bratislava, Venice, Budapest, Zurich and its own airport with just a few different through intercity services with easy connection between them all at Vienna. Frankfurt is similar. But you wouldn't extend an Edinburgh-London intercity service to Ramsgate or Portsmouth because those destinations need high capacity commuter services, not intercity services. Possibly Plymouth or Cardiff but only to join two routes for the sake of it, not for a fast through journey.
- The regional services would also preferably line up, and we are already doing this with Thameslink and Crossrail; you'd add routes like Oxford to Margate, Southampton to Norwich.
Having a very large high throughput central London station where through intercity services continue onto and terminate at Heathrow, Gatwick and Stansted would be very interesting though. This is the approach in cities like Munich, Vienna and Amsterdam - the airports are useful terminus points away from the Central stations, to which you can spread out termination points, add useful connection opportunities at the city centre station and shift any sort of layover and delay recovery time away from the centre.
You'd have to tear everything up and start again of course, and it's also a recipe for unreliability, not to mention that not even our electrification matches up north to south, let alone the routes!