Don't ever reverse this route ...
I flew BA CDG-LHR and got caught up in immigration hell then had to sprint through the UA terminal in my bare feet (note: never wear sandals when connecting through LHR and note #2: never ever bring 4 bottles of wine back from France when you might have to run with them through LHR).

Of course, the door shut 30 seconds before I got to the gate (Naturally the last one).