Did no one else notice that the
OP is from London (per the Profile at the left of their post)?
Yes,
it works one way in for US-based airlines (and maybe some other countries), but the opposite way on many airlines based in other countries.
I have flown BA (within Europe) while seated in an exit row once or twice, but I can't remember the policy, in part because there was no lack of overhead space, so I can't remember if I even tried to put anything in front of the exit row seat.
I do know that I was "scolded" (by everyone around!) for thinking I could put something under the exit row seat when I flew LAN on an intra-Argentina flight in the exit row. They've never heard of such a thing down there.