There has been a lot of misunderstanding as to what an ETA really is. I got this information from the original UK ETA website:
https://homeofficemedia.blog.gov.uk/electronic-travel-authorisation-eta-factsheet-february-2026/
This is an exerpt:
An ETA is a digital permission to travel – it is not a visa or a tax and does not permit entry into the UK – it authorises a person to travel to the UK.
What does this mean? It means if you plan to travel to the UK, and are from a visa waving country, you will need an ETA. They want to maximize the odds that you will be admitted into the country. Anyone attempting to enter can still be denied.
So, what if your plane is diverted to the UK, or you miss a connection? You get processed like any other person arriving. You didn't need the AUTHORIZATION to TRAVEL to the UK since you weren't planning on entering the UK in the first place.
One thing to note. If you are connecting at Heathrow or Manchester and don't plan to enter the UK, you don't need an ETA. However, if you are a connecting passenger at any other UK airport, you will need an ETA.