Originally Posted by smilinganddialing

Thanks but what about my specific situation where I have an intra-Europe arrival (T3) on BA connecting to BA transcon out of T5?

Your answer makes it seem like you perhaps misread my question as I have (a) an onward journey and (b) that flight leaves from T5. If (b) were to be T3 then I would be able to use the AC for sure so I think you might have thought I intended on using the AC prior to going through immigration and exiting LHR which is not the use case here.