Unfortunately there's no gray area... only reasonable and unreasonable BA employees.

Since the opening of T5 I have used the Concorde Room at least 6 times:
The 1st time I checked myself in showing the SFO-LHR First Class boarding pass, no problems there…. The problems started when, 5 minutes later I went back to the same attendant that checked me in to verify if the connecting flight (LHR-MIL) was on time. At that time, the overzealous employee stated pretty clear that I was not entitled to use the lounge because the connecting flight was in business (they don’t have First Class on those flights), therefore, even if my ticket was a full fare round trip in First and the reason I was not flying First on the remaining itinerary was their responsibility, I had to grab my stuff and move to the adjacent First class lounge
The 2nd time I had no problem since I was flying back to SFO in First.
The 3rd time they drove me insane taking more than 5 (real) minutes to understand that even if two of us (my wife and I) were flying First, while the other two (my kids) were flying in Business, we were fully entitled to use the Concorde lounge since it pretty well stated that if you’re flying in First, you can take in the lounge whomever you like.
I spare you the other three experiences where in one occasion I had to have the supervisor intervene in my favor and replace right on the spot the overzealous attendant.

In short, the lounge is beautiful, but if you’re not 100% entitled to use the CCR, than avoid the embarrassing situation where you have people waiting for you to get in, while you “try” to convince the attendant that you belong there (I HATE THAT !!!)

F.F.