I'm sure it will be on a Freeview channel soon enough. The only purpose for Britbox is to milk ex-pats, surely. Who in the UK would subscribe? 
I think they'll have a few UK-based greysurfer-types, so long as there's enough content which isn't available on existing Catch Up services (like BBC iPlayer) to warrant it.
And also new content not available elsewhere, like this new Spitting Image stuff. They'll hope to gradually build up a portfolio of such. It'll be interesting to see how well they do - they basically need to offer stuff which people want to watch and which isn't available elsewhere. Also what'll be nice for older people, is to see stuff which isn't full of American accents, which is what you tend to get on Netflix and Prime. Yes, both of those make a small effort to use British actors, but they'll tend to be in subsidiary roles and often playing the US stereotype of being an idiot/a 'Butler' role/ineffectual/villain. They do make UK-originated stuff as well, but very little in the scheme of things.
I think you're right about the ex-Pats being a large part of the customer base, but I'd be interested to look at their subscriber demographic after a year or two...assuming they last that long.
