P19 is the platform of choice for charters as if you arrive off the Windsor Reversible you can overhang it without blocking much. The issue with using more of P19 is the signal.
IIRC P19 was an emergency option to detrain a full Eurostar in the event the International station was blocked for any reason. Wasn't there also a back-up at Kenny O?
There appear to be effectively two starting signals, one 12 cars out and another 20 cars out.
I was on a trip couple of years ago with 'Braunton' with load 10 which came back up the Portsmouth direct then crossed to the Windsor lines at West London Jn to terminate in Plat 19. I overheard signalman telling driver he had 'stopped short' as when diesel loco came in to tow the consist out it hadn't cleared the starting signal closest to buffers. Steam driver offered to pull forward, but was advised not necessary, just be aware.
Have departed from P19 behind 70000 Britannia with load 12 plus a 67 on the back, so effectively 14 vehicles, which was routed direct to the down fast, rather than crossing over at West London Jn.
Need to check the connections at International Jn - when the upgrade works are complete will a railtour departing from P19 take out only 3 of the running lines (down fast, up fast and up fast relief), rather than the 6 that Britannia's move did?