I doubt very much that any good boarding school will accept a student into the final year of GCSEs as most of the material is covered in the first year. Most IB schools in the UK do GCSEs as well, unless he enters an American school, but I don't know if ASL is boarding (the other American schools are in the Windsor area, which might be a bit far out if you're looking for central LondoN). My school (all girls so not very useful for you!) won't accept students into year 11, but will allow them to 'go back a year' into year 10.