Go for it - don't be shy. Odds-on the crew will be lovely. it's usually (but not always) easier at the end than the start of the flight as they're more likely to be time pressed in preflight.

Enjoy - ask loads of Qs. Might get to sit in the skipper's or FO's chair and then they won't be surprised if you ask them to take a photo of you there. I think BA even have some sort of iPad app for this (although whenever I've done it I've just handed them my phone).

If you're asking before the flight, board promptly and ask one of the cabin crew as they come round (better that than the 'greeter' at the aircraft doors who will be busy). As you're upstairs in J then probably whoever comes round offering bubbly (although remember they still have to get that shortish cabin service done)

If aiming for after the flight then you have loads of time to set it up. But if you let them know sooner rather than later then also what sometimes happens (and depends entirely on what else is going on) is that the flight crew members may or may not stop by at your seat for a chat if they're passing through. Although my understanding is that if there are additional crew on 747 routes then you're less likely to encounter them in the cabin as I believe (and I could be wrong) that the flight crew rest area on the 747 is forward of the cockpit door and so they're all sealed in.

Someone will no doubt know the answer to this on here...

Anyway, enjoy.