Sharing my experiences in case they're helpful to others:

Departing from LHR: There was a scale at the entrance to the jetbridge, and it was used but not for every passenger. The first two people in line to board were told to weigh their bags. Both of their bags were just barely overweight (maybe only by ~0.2kg) and they were forced to check them. The gate agent got preoccupied with checking their bags, and the next large group of us were able to walk straight onto the plane without having our bags weighed. No idea if they ended up weighing more bags after that.

Departing from OPO: There was no scale or bag sizer visible at the gate, and I did not see gate agents make any comments about carry-on bag sizes. I checked in online and did not go to the check-in counter, so I don't know if they were weighing carry-ons there.