Separate company (LLC). Joint venture between ITV, BBC and AMC (et al). More info on their site if you're interested in the small print pages.
All programmes are apparently ad free. The biggest issue they have is acquiring rebroadcast / on-demand rights for catalogue -- it's a massive, massive, MASSIVE headache because I imagine they'll have to renegotiate entire swathes of content again for this new service. Rights negotiations is an absolute minefield.
The whole point of britbox, from what I've read on their site, is that they offer stuff not already available through BBC America rebroadcast / AMC (etc) syndication. It's a complementary service and it also offers ITV programming, so I imagine as we go on it'll offer Channel 4 shows too. Agreeing the rights and licensing periods will be what holds up new content appearing on the platform faster than it is.