CJB wrote

workers use Oyster. So the lack of cash change is a real problem. The
situation would be marginally better if the airlines sold Oyster cards
on board, or if Oysters could be bought from vending machines.

Oysters CAN be bought from vending machines. Heathrow may not have any
of course.

"2007-01-04 Oystercard card vending machines have been installed at
several stations (I have seen them at Liverpool Street and Euston).
They are cash only and sell an oystercard for £3 that then needs credit
loading at a ticket machine."

--
Mike D