Brighton is very academic & has high expectations. Maybe not crazy high like in London! Decided it wasn't the school for our DS as he puts enough pressure on himself without the school adding further. Wouldn't say its a strong performing arts school either, though it has good facilities.
Don't know your other schools, but would suggest you look at the state options first. My DC went to a fantastic state sixth form and saved us a small fortune.
We looked at Hurtwood! Crazy fees! Good for performing arts though. Also looked at Bradfield. Good boarding accommodation for girls but admissions staff and academic staff told us completely different things about admission and scholarships which meant we dropped them. They have a single assessment day for 6th form and if you can't make it there is no second chance. Academic staff said they would arrange another day but they didn't so DD went elsewhere. Head changed since we looked. If you are in London, will DS really want boarding out of London? Will he want to be away from friends? You could look at Stowe or Rugby.
King alfred school, portland place, holland park comprehensive - all worth looking into? Oh also Latymer Enfield? Also would your dd be interested in a college - city and islington? I don't know much about it (or others) - but again, worth looking at.
What are your dds' gcse requirements for 6th form by the way? Is it usual for the dteens to meet them; do they waive them if a dteen unexpectedly doesn't meet them?