I think realistically your reply should read more like "some people don't like the airline experience, [and] climate change [means] there is some demand for greener travel".
And in response, as @Ianno87 has already noted, there's absolutely nothing stopping those people from using current conventional and future HS2 services to travel to London and connecting there to current Eurostar services. Improving that experience is a hell of a lot cheaper than building a London bypass for what is really no more than a handful of people a day.
I'm waiting for the "but my granny with luggage doesn't like changing trains" argument.
