I dont know but assume international students are not eligible for British state schools.
There is a noticeable contingent from overseas looking for private boarding school places at 16. Look at the Student Room website thread on getting into Westminster School for sixth form. (Be scared, be very scared.) The same students will be trying Dulwich, SPS, Sevenoaks, Harrow and others. Lesser known boarding schools pick up even more, with some laying on, say, special English classes for Chinese students. This market has also encouraged the proliferation of private colleges. The fees are not insignificant so their families will expect students to work hard. Some of the results are impressive. However it will help if the year is seen as a way of getting to the next stage. Work hard, get the results and you are on your way.
I think for DC's cousin, the year was pretty much spent in limbo. He did not know many people other than his classmates and had stacks of homework which dominated each weekend. But he is now in the school he wanted to go to, over the transition, enjoying himself and with 10 A*s at GCSE well placed to apply for a good University.
In terms of the state sector if your daughter did go to a private college, she would presumably apply in the normal way for either a place at a school sixth form or at a sixth form college. Lots of kids move at this point. She will need predicted results. If these are good there is nothing to stop her getting a place at a selective school, perhaps a grammar. Catchments don't apply in the same way as they did at 11.
British Universities have always taken a lot of overseas students. However they have a set number of places for British students.