Just to give an additional data point, I’m posting from the F cabin of a 777 heading from LHR to the US. It’s about 5 hours into the flight and I just stood up to look around the cabin. Of 13 passengers, two are wearing face coverings. The others are either watching TV, working or sleeping (to be clear, the crew are all wearing face coverings). Apart from the initial announcement re US federal law requiring face coverings to be worn at all times, there has been zero enforcement.