As I understand, BA was more than a little upset at the need for transit travellers for ETA (and probably LHR airport itself). Would make transiting in UK an absolute pain for EU > LHR > US.
So they backflipped the need for ETA for transit pax.
But it does beg the question, what happens if you're a transit pax, arriving late into LHR for another late night departure and that plane is now delayed until the next day.
How does the airport, airline and government handle that. Force those pax to do a ETA on the spot? Given LHR isn't open overnight...