The UK government, famous for its good digital web services, recommends their official native app to apply for an ETA https://apply-for-an-eta.homeoffice.gov.uk/apply/electronic-travel-authorisation
You can use *web,* but the app lets you “complete your application quicker” and “get a faster decision”. From my extensive experience with UK Home Office procedures, I learned to jump into whatever hoop they present. App it is, I guess.