And at MAN, only within the same terminal.
I believe LGW intends to reopen an airside transit route "eventually"
EU > UK > US and vice versa is already a pain because the UK forces you to reclear security whenever arriving from outside the UK. On a Schengen > Schengen > US or US > Schengen > Schengen itinerary you generally only need to go through security at the origin.
LHR is open overnight, for passengers who cannot enter the UK whether by intention or by misfortune. If not already in T3, they are forced to take the airside bus there and then must sit in a defined area, usually by one of the low-numbered gates, until the morning.
An ETA is not required to enter the UK. Currently, the requirement is to have applied for an ETA before checking in for a flight to the UK (see
Apply for an electronic travel authorisation (ETA) "You can travel to the UK while waiting for a decision.")
In fact, at present the UK e-gates do not check whether you have an ETA, it is assumed that the airline will have done that. It is intended for the e-gates to reference the ETA database in the future.
There are always procedures for an emergency entry, for example if someone who cannot enter the UK has a medical problem that can't be treated airside. The ETA is intended to stop such people travelling to the UK in the first place, which is why they wanted it to apply to transit passengers too.
This is referring to the UK transitioning the Identity Card for Foreign Nationals (publicly referred to as Biometric Residence Permits) to Electronic Visas without making sure that the e-visa system actually works for everyone.
Similarly EU/EEA citizens who have obtained Settled Status from when the UK was in the EU, have no physical way to prove this status.
People who can't get their e-Visa to show up will therefore have problems travelling to the UK until the responsible government agency fixes things.