Actually, it is essential to keep hold of the First BPs. When I travelled through LHR a couple of weeks ago I simply presented my MAN BPs but the agent then asked if I'd travelled in First or had a CCR Card - I replied 'both'. He asked for the SIN BP or the CCR Card (this was an agent I hadn't seen before) and I said I don't know where my SIN BP was and my CCR Card is not to hand. He said the system simply told him that I'd flown in from SIN but not the class I'd travelled in. Fortunately one of the lounge managers came over to welcome me back and in I went.

I'm sure had the agent interrogated his system a little the information would have been there, but the moral of the story is ensure you have your First BP with you when you enter the CCR.