If you are unsure about when your ETA expires, or you are looking for some sort of documentation in case asked by airlines, this new addition to the UK ETA site may be very helpful. The UK does insist that actual documentation is not necessary because it is tied to your passport, but there are some reports of airlines asking for it anyway......
As a reminder, an ETA is good for 2 years or when your passport expires, whichever comes first.
I applied for mine as soon as they became live so I have the earliest expiration date possible for active passports which is January 8, 2027.