I often (well, not often, but whenever I pass through) wonder what the thought was when they were planning the Eurostar lounge at St Pancras. It has always seemed entirely too small for the passenger flow it gets, which seems to have been slightly worsened by Brexit as people rock up even earlier now, but surely when they were designing the space they must have known that it would be inadequate to the numbers that would be expecting to use it?
Was it simply money talks? They could fit some extra retail units for the station by keeping the lounge to the minimum so passengers for Eurostar lost out?
It is a bit of a mystery to me as well. The Additional Protocol to the Sangatte Protocol (essentially the law that regulates the juxtaposed border controls) was signed in 2000 and brought into law in 2001, so the design of St Pancras should have included a large departure hall and a very small arrivals area. The customs area on arrival could have been dealt with by simply having a small area with a couple of scanners and desks, and there was certainly no need for a very large arrivals hall.
Even if the initial design work took place before 2001, it could easily have been changed during construction to take into account the fact that there were now no border controls on arrival. The only logical thing I can think of is that there was some uncertainty over whether the Sangatte Protocol would survive, and hence it was desirable to keep a large arrivals area available in case of need. Certainly, the design of the original arrivals area makes zero sense in the context of not having controls on arrival.
It may be worth considering converting the shops directly next to the departure area into a bigger waiting area, and see if they could squeeze one or two extra security lanes in there
I would argue that the best solution would be to place border controls at the current entrance to the Eurostar area. You could set up the automatic gates there as well as the manual ones for the French controls, then have a second layer behind that for the UK exit checks, ticket gates and security carried out by Eurostar. With the entire area transformed into one large departure lounge, you could configure arrivals so that they exit directly into the retail space at the...west? side of the station.
That should provide enough capacity, especially if the French entry controls have plenty of e-gates and manual controls as required. It would involve taking away the taxi drop-off entrance, but I suppose that could be retained as an entrance for Business Premier customers with access straight into the lounge with border controls. The ticket office would have to be relocated, but do they really need that much space in the era of e-ticketing? A small retail outlet would probably do.
Perhaps it would also make sense to remove one set of escalators leading to the platforms as well in the departure lounge? They don't need six escalators, three would suffice, and this would provide some more room.
The problem will arise when 2 near-simultaneous departures occur...
Maybe with what I propose above, they could divide the departure lounge into two, one for each departure?