So I'm just now applying for our UK ETA's, do I really need these just to get out of LHR, go to the train station, and depart for Paris by train? Second question, it says 16 GBR for the ETA each, but it's forcing me to pick a processing option with a minimum cost of 79 GBR for each pass? This adds 200 GBR cost to the trip that I hadn't anticipated if accurate?
Sounds like you’re not on an official site - there shouldn’t be a processing charge for the ETA. What’s the domain name of the site you’re on?
Here’s the official site. It gives you the option of downloading the app or applying on the website. It will always have a gov.uk URL.
And yes, you need an ETA. If you were simply transferring airside at Heathrow to a flight to Paris then you wouldn’t. But you are entering the UK, even if you’re just spending a few hours.
(Just as every British visitor has needed an ESTA to enter the US for many years now, even if we were, for example, getting straight on a train to Canada.)