Except that in practice, check-through will be at the whim of the agent you check in with.

I'm not saying you will or should be denied, just warning that an agent may not know the rule, his supervisor may have given him specific albeit incorrect instructions to the contrary, he doesn't know which buttons to press to get the job done, etc.

I'm just suggesting that if check-through is important to you, you might best be prepared to argue the point. It's said there's a statement on the OW web site giving that check-through policy (as correctly stated by jridge - I haven't looked, but there's supposedly a very positive statement that check-through to another OW carrier even on a separate ticket will be accommodated) and I'd carry a printout of it.

(For your heads-up, there are current discussions in the BA thread about painfully long lines getting through security at LHR T5, even for FastTrack pax. Just one more thing to make flying less fun than it used to be. )