There aren't many PE seats (just 38 on the Gatwick - Orlando route). I've flown PE quite a few times now (around 16 flights at a guess) and the cabin has always been full. The upside of that is that we've been upgraded to Upper Class a few times.
Friends of ours were in the same situation as you in October. Their 22 year old son decided to travel with them at the last minute and they were able to get him into PE on the way out, but not homebound. They kept trying right up until the last minute, a couple of times during the trip and at Downtown Disney check-in, but without any luck. However, it was a busy time.