As you are on the last flight leaving Gatwick ( on VS that is

) you do not have to be there when the doors open ):
That said, there will have been a lot of flights through earlier and if your wife is set on having a treatment then you need to give yourself plenty of time
Mrs Buns and I normally aim for 8:30 - 9:00. This allows us to book a treatment, then sit down and have a full breakfast, relax and take in the surroundings and if necessary go and look at the shops in the Terminal. If the flight is leaving on time expect to be called about 12:15
What I can say is that the staff there will take good care of you and will make you feel special. Make sure they show you around when you arrive
buns