Originally Posted by
gnomeza
Less out of the way than Milan, but can you book a fully flex leaving from T3 to take advantage of the Qantas lounge?
You are entirely overcomplicating it, the OP is arriving on an inbound flight from the USA so could transfer over to T3 and make use of the lounges... before later returning to T5 Flight Connections.
All documented in this thread:
Switching terminals in LHR for lounge use