Looks like it got caught up in the general delays that LHR departures have been experiencing in the last couple of days (FlightRadar24 showed that most morning departures had significant/major delays).

The incoming flight (OS 451) actually landed on time (0822 vs 0820 STA) but took off 60 min late. Thanks to padding it came in 35 min late but obviously that was enough for you to miss your connection.

On Wednesday evening Heathrow experienced severe disruptions (some relating specifically to BA, others more general) and this had major knock-on effects into Thursday. Possibly the delays today might still be related to that.
You could try also asking on the BAEC forum as the guys there will be able to give tons of info on any current issues at LHR.

https://www.bbc.co.uk/news/uk-england-london-44885185