In a bit of a nod to a thread a bit further down about how BA wins because of direct flights, I'm not sure I'd be electing to take an indirect flight when direct options are available just to get an exit row seat on a short haul feeder flight. Especially not one that connects via Heathrow.
I know what you mean. I do always think twice about booking with BA and having to switch airports. But the alternative is booking flights from Europe to the US with an airline I don't have status with and I don't fly enough anymore to get status. I could still fly ex-LHR to the US but that would involve a separate ticket to get me direct to LHR (and I've gone that route before and had to pay a bunch of fees when OW flights were delayed and caused a misconnect to the non-OW flights). So I accept the inconvenience of switching airports to stay loyal to OW and also stay on one ticket (OK, the lounge access doesn't suck either). I live in Europe and fly back to America a couple of times a year.
I think the agent was being truthful. The staff here is contracted and she really did seem to care that my seat was changed but said it wasn't available. Maybe CE did go back further. I was in 10D on an A319 (I think) but didn't think it usually went that far back (and was still showing ET just before T-24). She said it was 'offloaded' so I assumed someone doing online check in after my attempt was able to snatch it up.
Oh well, at least I have lounge access!