The UK ETA transit requirement depends on how you transit through the United Kingdom.
If you stay airside at a UK airport and do not pass through UK border control, you generally do not need a UK ETA.
However, if you need to pass through UK border control for baggage collection, terminal transfer, leaving the airport, overnight stay, or any other reason, you may need a UK ETA depending on your nationality.
A UK ETA may apply to travelers who enter the UK or pass through UK border control while transiting, so it is important to check your ticket and transit route before traveling.
If you are not sure whether you will pass through UK border control, check with your airline before your trip.