Indeed, one is outside the front door to the station building, the station building always seems to be open later into the evening though I've not had to go round the outside for a while! I know they have more staff now due to the lift needing someone present, but unsure what time they are there until!
Midnight I believe, according to the poster that's now gone. The other benefit of this is that you no longer (well until after midnight) have to walk all the way down platform 1 to leave - or indeed, buy a ticket, then walk all the way down to the end of the platform to enter via the night entrance/exit-cum-fire exit.
There's a TVM at the front, which generally works okay. Then two more inside. The fourth one that was on the other side, near the original night gate, has gone and wasn't relocated anywhere.
Be warned, the TVM inside the ticket office - to the left of the windows - has a buggered touchscreen. It needs fixing or replacing. I reported it ages ago, but nobody cared. If you try to select a station via the A-Z you'll find it inserts spaces when you aren't near the screen, and selects twice or even three times when you enter a character.
Collecting a ticket you ordered online can take ages.
I'm amazed it hasn't been fixed in months. Basically, don't use it unless you're just going to select a ticket from the front page, don't have a railcard or need to select multiple passengers.
Whilst this is now moot, I thought I'd reassure you that there is indeed a ticket office within the gateline at Kings Cross; it's called an ‘excess fares office’, and you can see some photos of it
here. It's located within the original station wall alongside platform 8.
But it's only open at certain times late at night isn't it? I don't recall seeing it open most times I've gone by.
All I see is the staff of the Harry Potter shop going into the nearby store room to bring out more merchandise. Maybe they can sell you a ticket if you say you arrived at platform 9 3/4?