First of all it should be a crime for them to assign appointments so early in the morning for people who live far away from london, but once you get there and get through security, it's easy as hell. The interview itself took like 2 minutes.
All the staff were nice and helpful etc etc HOWEVER if by any chance any other transgender person should be going through this process: the guy I saw had never seen a gender recognition certificate before so I had to explain it to him (while other people could hear....). Just be prepared to have to feel really awkward about explaining the name change and such.
Also the questions I got in the interview were: where does your wife live, when was the last time you went over there, has she ever been here, does she have her own house/apartment, when did we first meet in person.
So a pretty simple process, honestly the worst part was having to wait in line in the five million degree heat outside for like 10-15 minutes