Another data point which shows a possible change in allowances.

I had a 6 hour layover at LHR this past weekend, arriving on QR in Terminal 5 and connecting on BA to INV from Terminal 5. Given the long layover, I was keen to spend some time in the CX and QF lounges in T3. Via Flight Connections, I arrived into T3 by bus, and attempted to go through the transfer security to enter the terminal. When my boarding pass was scanned, it gave an error message to the lady scanning, saying that I was departing from T5. I explained that I wanted to visit the lounges in T3 and that I understood that I would have to reclear security for T5 to catch my flight. She wasn't aware of this being possible so she phoned her supervisor, who phoned the terminal manager who denied me access to the terminal. I ended up eventually speaking to the supervisor, explaining that I had done this before without trouble, but she insisted that the terminal manager said this was not allowed, apparently due to too many people doing this and subsequently missing their flights. She said it was essentially a BA issue that because my BP is for T5, there would be nobody taking "responsibility" for my presence in T3.

I further spoke to the BA agent at the transfer desk to try to see if there was some way to get in, and he further spoke with the terminal manager and security staff, and again said that it was not possible, and this change had been put into place pre-Covid and any time that myself or others have been allowed through was due to a lenient terminal manager.

As there is conflicting information from my previous experiences and others on here compared to what I had on Sunday, I asked who I could write to determine what the actual rules are regarding access. The security staff said it was a BA issue and to write them, and the BA staff said it was a security issue so I'd have to write Heathrow itself.

Does anyone have any further info or who is actually correct to contact? Or whether the rules have changed or if we have just been lucky over the past few years switching terminals?

I ended up spending my layover in the BA First lounge, which is heaps better than the last time I visited there about a year ago, with some tasty cahmpagne, so all was not lost.