Heathrow has always operated badly.

For example, they don't do one-stop-security at all for transit passengers. Fly from UK/EU/North America to Heathrow to somewhere else? Heathrow will re-check your carryon luggage and put you through the metal detector upon arrival as if you came in off the street. The only exception is if Heathrow is your final destination.

Major EU and North American airports don't waste their time with already screened passengers from trustworthy countries, but Heathrow is too disorganized to organize itself to save all this time.

https://blog.aci.aero/dont-stop-me-now-how-one-stop-security...