It will never brake if you're not using autopilot! On the flip side, if you're using autopilot and press the accelerator when it detects a car infront (within the distance of the adaptive cruise zone) a warning comes up to say that it won't auto-brake if you're actively pressing the accelerator. If you set the active cruise to 77mph and the traffic infront slowed to a stop it would slow down too.. it works well.
Top tip: remember to turn autopilot off when you exit on to a slip road as the car will want to accelerate back to 77mph once it detects no obstacles
To be honest, if you're thinking of cancelling based on something that may never happen then I'd cancel now. I travel 20 miles to work which is all-motorway and use the autopilot feature 24/7 when commuting. I've done 1200 miles including a 500 mile trip and it's never once phantom braked or done anything screwy. I wouldn't try using autopilot on narrow streets or rely on it to be 100% perfect in urban environments as you will be disappointed but the autosteer (keeping you in lane) and the adapative cruise (autopilot) are amazing.
One pedal driving is on by default... it's nice to be able to lift-off and feel the car braking. The regen effect disappears completely above 90% charge (the battery won't accept regen because it'll fill the cells) which is something to bare in mind if you get used to it then do a long trip.