Yeah, the entire travel story of the summer is that every airport is kind of screwed, it's not remotely limited to Heathrow. According to this source, Heathrow actually outperformed every other London airport except Stansted in terms of percentage of flights successfully departing in June.
https://thepointsguy.co.uk/news/worse-uk-airports-cancellati...
And of course Manchester had some well publicized issues, and outside of the UK AMS and CDG had massive problems as well. I mean, KLM flat out stopped flying ticketed passengers into AMS for a day to try to recover.
"At the end of Saturday afternoon, 4 June, KLM was regrettably compelled to decide that passengers at European destinations would no longer be allowed to board flights to Amsterdam."
https://simpleflying.com/klm-suspends-all-flights-from-europ...