I do not get the hysteria over needing an ETA for airside transit.
If a flight is £100 cheaper via LHR than say AMS or FRA, then a £10 online form is hardly a big deal.
The fact is that airside travellers would be able to head to the immigration desks and claim asylum. The logic is the ETA requirement will decrease the likelihood of that happening.
I believe Australia and Canada require ETAs even for airside transit. US requires you to go through customs each time.