From the Ba website I gather that:
*BA Gold Card holders arriving on a Longhaul sector in any cabin
* Any FIRST or Club World Passengers
can use the Arrivals Lounge in T5 Landside.
I suspect if “all the others” could use it it would be getting too crowded at times and I think this is a fair enough policy.
As for the Arrvials Lounge cards which should be given out in FIRST and Club World into LHR they are for information purposes only so eligible Customers can find out where it is, yet you still need to show your F/CW boarding pass or your Gold Card and boarding Card so Lounge Staff can verify that you are indeed eligible to use thie Lounge.
I always found the Lounge Staff in T5 I have come across courteous anf driendly but I guess there is always one ro two who spoil it.