Stranded Pax
I think it depends very much on the route as to whether the numbers stranded at each end were roughly equal. On routes like UK to Sharm-el-Sheik you would expect almost all the traffic to be originating in and returning to the UK, and the eruption happened just at the end of the Easter school holidays so there would in any case be more people expected to return than travel outwards. On this sort of route a lot of passengers expecting to depart the UK would have given up altogether and taken the refund or rebooked for later in the year.
Inbound tourism to the UK is all-year-round, but April is by no means high season for it, especially from North America.
On routes with more business and "visiting friends and relations" passengers it would be different. Here the problem in the early stages of resuming flights would be getting the stranded passengers to the airport once the pool of people found hotels by the airline had been dealt with, they might need connecting flights from elsewhere which hadn't themselves restarted yet or were full, or lengthy journeys by land transport so couldn't get there in time to catch the first flight offered. Anecdotally there were indeed empty long haul seats outbound from major hub airports such as Heathrow and Amsterdam. It would be helpful to the discussion if this could be confirmed or denied by people who flew or handled such flights in the last few days.