If it were only an hour or so, I would suggest a Costa at one of the terminals. But for nearly 6 hours (even assuming that the first flight isn't early and the second late), there'd be time to go into London, see a few sights and return! And that leaves jet lag after an overnight flight out of the equation. It's sheer madness to meet at the airport, unless you are travelling by car somewhere difficult to get to. If that's the case, then take a day room at one of the airport hotels.