Just flown LHR-LAX-LHR in J.
At LHR, the agent didn't even want to weigh my roller-board and carrying on that and a small rucksack was no problem.
At LAX I saw the people in front of me having their carry ons weighed, so I emptied a few things out before check in, and it came to 7.7kg which was fine. I then put them back in after I checked in.