Initially denied passage at T3 screening
I'm at a lounge in T3 now, and have a tale to tell.
We arrived at T3 this morning on a CX flight, connecting later today on a BA flight out of T5. We already had our BPs for the BA flight so we headed directly to the T3 connections screening area. The BP checker at T3 screening denied us access stating that "security" would not allow pax with T5 BPs into T3. I protested that we simply wanted to use the lounges in T3. She stated that we would need to get a "pass" from the airlines if we wanted to use the lounges and directed us back to the airline service desks.
We walked back to the service desks and spoke with a BA agent who stated that we should have been allowed through. She kindly offered to walk with us back to the BP check area. Once again, the BP checker stated that "security" would not allow pax with T5 BPs to go through, however, this time she stated that we could go through with either a "pass" or with a lounge membership. Thankfully, I had my AA lounge membership card with us, showed it to her, and she finally relented and let us through.
Something did not seem right about this entire process. The BA agent was genuinely perplexed at the idea of needing a "pass" and did not know that such a thing existed. She also stated that she had never seen pax denied entry to T3 when trying to use the lounges. Also, the entire crew of line minders/BP checkers at the T3 screening area were very young. The "senior" person that the other three checkers all deferred to appeared to be in her mid 20's. Not saying that youth is an indicator of poor job skills, but possibly that they have limited experience with other than typical situations.
On a happier note, we did find the 'double doors with button' passageway back to the arrivals stream - it is at the far left end of the security screening area, immediately after you collect your items from the x-ray machine belt. Also, the passage we took from the screening area led us to point "B" next to the Kurt Geiger store, so that confirms my earlier supposition. Ta to all for your guidance.