Regarding passenger flow, a few data points:

Arrived T5 LHR Saturday AM on the 19th at 630'ish from BOS. Pulled direct to a stand where stairs arrived promptly. The first bus to T5 Immigration/Arrivals was for First and Club World passengers. I didn't see if there were other buses in the area for WTP and WT passengers so hopefully they didn't have to wait too long. I was through Immigration almost immediate (US passport, scanned and through in minutes upon arrival). Arrivals Lounge was not crowded at all.

Return to BOS yesterday March 20th: Used VeriFly to confirm docs and covid test needed to the USA. Online check in the evening before worked fine and few lines at T5 Departures. I walked out of my room at the Paddington Hilton at 7:40 AM and was in the Galleries South Lounge in line for a bacon roll by 8:55 AM. Crowded lounge with mostly solo seats to be found. Several families were walking around looking for space that simply didn't exist.