I'm flying PHL-LGW soon, and I was just wondering, how are the GoEnvoys selling?

The Envoy availability for my flight shows at least 9, but the seat map has just a couple of empty seats. I assume open row 1 seats are blocked, and it looks like row 2 starts all blocked on future flights, so the seat map isn't much help. If I decide to buy the upgrade do I have to pounce at the 30 hour mark? Or can I still be thinking about it when I get to the airport? What has your experience been? I'm guessing demand may not be huge as coach looks to be sparsely booked, OTH reclining on a red eye is nice.......