Here's a suggestion if you want to get into the First Class Lounge...

Assuming that you can't get in with a transcon First ticket or aren't on a transcon, book a full-fare first class ticket from JFK to LHR on the payphone outside the lounge. Go into the lounge and have the agent look-up your e-ticket, but tell her you don't want to check in, you'll take care of that in JFK.

Enjoy the pleasures of lounge. Leave a bit earlier than you would, stop by said pay phone, cancel your JFK-LHR ticket, and head to your boarding gate!

Same goes if you have access to a club, but only with a same-day ticket (Amex Platinum for CO/NW/etc for example).