Electronic travel authorization

If you are flying with all flights on the same ticket, then you dont need a UK ETA , since you won't need to clear Passport control there. You can remain airside, even if you need to change terminals at LHR. Look at the Connections section of the LHR website for more info.

If you have checked luggage and are connecting to a flight on a separate ticket, you would need an ETA because you would need to go through Passport Control in order to access your luggage and check in for your next flight.

There is no ETA for the Schengen zone or EU. At some point next year the ETIAS requirement will come into effect.

Is there any scenario where this documentation would be necessary?

If there were an airport or airline fiasco that precludes onward travel that day and you had to go to some off site hotel. Extreme, but in the realm of 'could happen'

Is it worth it to rush order this authorization or is this an imaginary problem? Thanks!

No need to 'rush order'. Usually the UK ETA takes only minutes to get approved ( once you do the application)
https://www.gov.uk/eta/apply

The problem is not imaginary, but highly unlikely.

The ETA is only for the UK.

Should something happen, don't fear that you will have to sleep on the tarmac. The ETA is basically a pre-authorisation that allows you to travel to the UK. If for some unforseen circumstance your plane is canceled and you have to enter the UK, you will get processed by a Border Force officer. You probably won't be able to use the egates but it's doubtful you will be denied entry.

They aren't exactly doing a deep background check. My ETA was processed in literally 30 seconds after application. The computers run your name and passport information, look at any criminal record or previous denial of entry, and it makes a decision. That's all it is. It's a way to save time at immigration and to collect some money. It's not a visa

By the way, an ETA doesn't guarantee you admission into the UK. As in the past, immigrations officers can deny you admission if the feel you will be breaking the law.

if you're a belts and suspenders type, there IS always a possibility that you will be stuck at Heathrow. So why not just get the ETA so you can leave the airport if needed or desired? That's why I got mine last week.

Unlike other airports, I don't believe Heathrow has a hotel that is actually inside the grounds so you would have to leave.

if you're a belts and suspenders type, there IS always a possibility that you will be stuck at Heathrow. So why not just get the ETA so you can leave the airport if needed or desired? That's why I got mine last week.

We are transiting LHR next week. Both my wife and I got our ETA's a couple of weeks ago in the off chance that our continuing flight is cancelled and we have to overnight in London.

We also had a trip via Heathrow to Milan this spring. Being, a just in case person, we paid for the ETAs. It was inexpensive and only took a few minutes. Just in case…

You don't need an ETA to leave the airport. It's for travelling to the UK. The clue's in the name.

Frank II's understanding is correct.

There are long standing procedures for people "stranded" overnight at UK airports because of cancellations etc without a visa & these have been extended to those without an ETA. People are processed by an officer and either admitted so they can go to an hotel or are refused & have to stay in a holding area set aside for the purpose in Terminal 3.

For ETA eligible nationals admittance is most likely - only if they are inadmissible anyway (eg criminal record) are they going to be turned down.

Thanks, these responses are very helpful. Appreciate it!