I was on an AA day flight JFK-LHR last year that was going to be delayed because of a delayed pilot,

It became quite tight on the timings and I got the impression that AA would rather have delayed the flight than have to pay the LHR fee and use up one of the night time quota slots.

There was also mention about public transport shutting down as well which wood have affected lots of people.

we did take off around an hour and a half late and landed only an hour late. I wouldn’t say boarding was rushed but lots of encouragement was given to get people to board quickly and get settled.

The BA and VS flights are around 8am so have a couple of hours leeway over the AA which is a 10am departure.