As some one who works in Operations near LGW, I was keeping an eye of the delays into Heathrow all day, by Lunchtime, 95% of BA's flights had ATC slot delays of over 45 minutes, most over the delays where 60-90 minutes and the longest I saw was 158 minutes!
Once aircraft get these kind of slots, there will be cancellations, there will be conjestion, departing aircraft also get delayed by high winds, which means aircraft get stuck on stands.
This situation is well outside BA's control, at least you got to London, not everyone was so lucky!