Don't assume every flight is full, might only be about 25% full on avg. Does Heathrow take no flights from elsewhere in UK (like Belfast)?
All sorts of people are simply returning "home" after international departures they decided to make 3-24 months ago.
Someone on here (Uk resident) posted about her partner who is an oil-platform engineer & regularly goes off to places like Malaysia for 3-4 week stints working there. That is common that industry (long stints far from "home"). There are oil/gas engineers regularly working in UK who have families they live with in Netherlands, Belgium, Denmark, etc.
Need an organ for transplant, some types of legal document, technical scientific equipment, biological tissues, some medicines? All those get escorted personally.
A large % of those landing at Heathrow are in transit going on to Ireland or France etc.
The list of people exempt from quarantine rules includes all sorts of business people, technicians, scientists, engineers, health care professionals. Until recently, journalists, too.