The ACS schools have rolling admissions, you can apply and start at any time, provided there is space in the appropriate year group. There is no application deadline unless you're looking for a bursary.
Despite their names, they are international schools, not strictly American schools. And as far as I am aware, they follow the IB curriculum rather than any American curriculum.
I gather that the three different schools have their individual, distinct characters. E.g. ACS Hillingdon has a lot of diplomat families, ACS Egham has a larger British contingent. Also I believe the curriculum isn't exactly the same at each school.
To me they do have an 'American feel' to them in terms of the security at the gates, lockdown procedures and such; also, no uniform.
If they like this corner of Surrey (it is convenient, for airports etc), but not the ACS schools, they could also look at TASIS in Thorpe (near Egham).