The need to expand is clear, what is not clear is how, either commercially or in terms of space.
There are some possibilities in St Pancras:
1) Turn the platform area (at the end of the tracks) into an arrivals area. You'd only need to put up a few screens and create a small area for Customs, with a secondary check area that can be located almost anywhere else. This would free up a large amount of space, especially if you removed the escalators at that end of the platforms.
2) In parallel, transform the existing check-in area into a joint check-in/security and passport control area. With the rise in automatic border crossings, it should be possible to create some infrastructure that will funnel people through without requiring huge amounts of space. ETIAS registration machines, if required, can be placed somewhere else within the station. The other entrance can also have check-in machines and border control.
3) Extend the departures lounge towards the National Rail ticket office and remove the cafes there.
It shouldn't be too costly to implement these changes, and it would provide quite a lot of additional space. While it would be an architectural disaster, I'd also give serious thought to taking over the public space next to the platforms, as there's quite a lot of room there to put in an additional departures lounge.
I'd also think about things like "trusted traveller" programmes that would allow for them to bypass security, reducing the amount of space needed.