Welcome to Flyertalk advani, welcome to the BA forum, it's good to see you here, even if the immediate posting doesn't fall into the category of good experiences. I very much that you will continue to post here.

Hopefully this won't happen to you again, but if you do see a big queue at Flight Connections, and assuming you are eligible to enter the UK (plus the UK Border queue doesn't look too bad), then you are best to go landside to Zone E to get sorted out, there are rarely queues there.

I'm not surprised about the Arrivals lounge in one sense, namely they are particularly strict on access at that lounge, more so than CCR, and it's unfair to impose yourself on staff, who are under the cosh about this, and if they had to call the manager for every "try it on" they would be on the phone constantly. Someone seems to try every few minutes. However in another sense, if you are Gold and arrived on a Joint Business codeshare from USA then I'm trying to work out why you were denied entry. Maybe there is some other feature involved here, can you recall what the specific issue was?

In a way I don't think anything wrong happened here: the staff did the right thing to you (with the caveat mentioned above), the management injected some commonsense to the situation. And frankly the First Lounge in T3 would be the best place to be anyway.