For those flying from the US and connecting through Heathrow, sharing some airport security info from my brother who connected through Heathrow last fall, to maybe help avoid a bit of added delay -
Unlike many US airports, at Heathrow all liquids of any kind must be removed from carryon luggage and put in a clear plastic bag when going thru security.
They had a flight from the US to Heathrow and connecting flight from Heathrow to another European city. Even though the inbound and outbound flights were in the same terminal, they were routed back through security.
In the US with TSA Precheck, in many airports liquids can be left in your carryon bag. When they had to go through security again in Heathrow they had removed their obvious liquids, but did not remove things like toothpaste, eye drop, deodorant, etc. The bags got flagged and they had to go thru secondary screening that took another 30 minutes because of the backlog.
Definitely suggest taking out anything that might resemble a liquid or gel and put it in a plastic quart bag.