Twyford is (apparently) a kinda strict school, and there are 4 sets of exams each year. William perkins idk.
Look at each school's stats, may be kinda hard to find on the website but check. Also, go to their open days, since you can have a feel for their school and what it's like.
I stayed at my own school bc the teachers knew me and i knew what its like and how its set up etc