you need to be in the office as a junior dev because you don't know what you don't know and you can learn a lot through osmosis

Learning by osmosis is just not a thing. You can't just smack a junior in the middle of a team that doesn't effectively do remote, but a team with a good remote culture should certainly have no issues with a junior being remote as well.

how does a junior dev know the difference when accepting a job offer? (aka you don't know what you don't know)

I have mixed feelings on this, it seems to me it is doable, but it requires 2 important things: 1. The Junior has worked remote before. If you get someone where this is their first remote job, unless they're incredibly organized and have a remote strategy they will lose socialization and much more. 2. A ton of work on the manager's part. Mentoring sessions, checkins, very active communication channels etc.

How remote? 1 visit every 6 months, or 1 day a week? And what type of dev - software, or something more BBC specific involving actual broadcast?

1 visit every 6 months based on location and software

Maybe. I wouldn't recommend it.