I came home from my last business trip in early March. I took a couple flights within Europe that were a bit emptier than normal, but still probably 60-70% full. I connected in Heathrow airport for my flight back home. LHR was still reasonably busy: security had lines, terminal shopping and seating were crowded, and the lounge I visited was near capacity.
My flight back to the US was a different story. When I checked in, the BA seatmap was crazy, showing maybe 25% of seats full. And that's what it was on the plane. Too bad flights from Europe to the US aren't overnight, as everyone in economy could have stretched out to sleep across 3-4 seats.