They generally hook them in for week 39 unless you have a medical reason to do it earlier or later. They do 39 not 40 (unless requested) as it’s means you are less likely to go into spontaneous labour and therefore need an emergency section. You can
In terms of having a pre op, getting all the info /medication you need and having the swabs done for the operation id say mention it at your 36 week apt, obviously I’m sure you can ask for one after this you’d have to contact the midwife for a consultant appointment when you’ve decided x
Not technically, you could go into labour and walk into hospital and say you want a c section. For planning purposes it makes it easier to mention it asap in case there are a limited number of slots