For the ten billionth time, BA have no bloody choice but to axe the Shuttles as Heathrow is maxed out in normal operations and there is no leeway when the weather gets involved. Hence when fog / winds hit you have options :
A) Land the inbound Hong Kong that has been holding at Lambourne for 45 mins
B ) Divert the above flight with 350+ pax
You choose to land the above but to free a slot you need to consolidate as many of the domestic shuttles together to free up space in the sequence as ATC are under pressure
Result? 350 long haul passengers get into the right airport and your domestic passengers get home late.
Or there's option 2 which is try and run the normal schedule which would see the long haul fleet and their crews scattered to the regions and tomorrows program damaged even more as the crews that were down to operate them are in Glasgow, which oddly enough is where you wanted to be! Result? You have 350+ long haul passenegers not going anywherethe next day. Now multiply that by the number of long haul inbounds taking into account domestic outbounds and passengers carried.
NOW can we have a third runway?
The same capacity constraint is NOT applicable at CDG / FRA / AMS. They have what is known as "room to breath".