Starting Feb. 25....
The price of a UK ETA will rise to 20 GBP.
Enforcement of having an ETA will begin and if you don't have one you won't be allowed to board your plane, train or ferry.
Whether or not you will now need one to transit is questionable. An email sent out by the US Embassy in London says you need to transit but I couldn't find that fact anywhere else.
However, a blog put out by the UK governent says transiting passengers who do not go through UK passport control don't need one.
https://homeofficemedia.blog.gov.uk/electronic-travel-authorisation-eta-factsheet-november-2025/