The most appropriate analogy I can think of is BA saying that if your flight is from T5B you can use that lounge, but not GF in T5A. So you can hang around shops and restaurants beside a lounge you can't use, or you can get lounge access - there not being a way to move back to T5A.
I'm so used to the situation now I don't really mind, but even if BA can't let everyone access GBL (if it's a capacity concern) then at least allowing those transferring from another flight would help - though appreciate it doesn't address the inconsistency / research points raised.