Scottish universities might well be more willing to be flexible with admission with US high school qualifications but as others have said good English universities want very good AP grades for admission.
I looked into the fees issue for a school-leaver from outside the EU and I believe that even the local colleges (equivalent to community college) where one could take A levels/an HND/BTEC would charge overseas fees, though not nearly as steep as university. For example:
http://www.ccn.ac.uk/ccn3/overseas/fees.htmMy US cousins, however, worked out that UK fees plus air fares were cheaper than private university fees in the US (but as my uncle is a professor the family rates were cheaper still).
I also have several colleagues who have worked outside the UK and despite being UK citizens they either need one partner to come back to the UK when the oldest child is 15 or to pay overseas fees. One family I know did the former, with the wife commuting termly (to Africa!) and another family have worked out that the UK tax they save will pay the fees.
For your younger child 2 years doing A levels and then maybe a year out will enable them to gain residency. They may be able to start A levels without GCSEs but most university courses will require GCSE English and probably Maths too so perhaps doing those at the same time as the A levels would be helpful, to avoid universities who don't really understand US qualifications getting confused about the switch in qualifications.
