Except when slot restrictions have to be imposed due to BA's own incompetence. Such as the event on 10/11December. BA wasn't prepared to enable de-icing of its aircraft, so capacity had to be restricted to avoid Heathrow grinding to a complete halt. On top of that BA's crewing systems got completely out of sync because they are clearly not fit for purpose (meaning crew were available for flights, aircraft were available for flights, but BA couldn't match the two together). But of course BA doesn't admit to any of this and just claims it was due to adverse weather to avoid paying any compensation.
So there's a heck of a lot that an airline can do.
I absolutely agree in that circumstance. I'm not one that will walk around and blindly say BA do a good job on everything, when things go South with BA they tend to do so very quickly. I wonder what the cost to them would be to implement a procedure where A/C are de-iced prior to crew arrival on S/H wave 1 automatically if overnight has had precip & sub 0 temperatures. It's a system used in other countries and one that is very effective. Especially with the type 4 holdover times being so long. I'm not familiar with the fluid types and step procedures used at LHR, I'm sure Airprox will be along shortly to let us all know
