On the other hand, do we have any documented cases of BA not accepting responsibility, particularly in situations where a bag is checked through and you have boarding passes? My impression is that if you miss a connection because of a delayed flight, it is quite easy to get transferred onto a later flight, even on separate tickets.

What may be more tricky is a cancelled or sufficiently delayed first segment which meant that you could not get a boarding pass for the second segment. I don't suppose that reservations would help, but it may be possible to get transferred onto a later flight when checking in for the first flight?