Originally Posted by
KARFA
It’s pretty much destined to end in disaster.
Firstly if stepping on the transit at B you will have to go to C first then all the way back to A stopping off again at B to pick up arriving passengers. You would need to exit through immigration on getting to T5A or go through connections. Prior to scanning in at security in either case you will have to find a BA agent to reset your ready to fly marker - not trivial and will require a little bit of time and some explaining. And you need to be scanning in at security no later than 35 minutes before your scheduled departure time.
It seems like at that point one should just give up / hit the reset button, go through the border (assuming no issues there) and head to the First Wing (if able).