Colchester Royal Grammar School has no catchment area. Bits of Colchester are quite pretty; I was thinking that only yesterday as I sat in a traffic jam alongside St Helena School. It also gives the lie to the idea that Essex is flat
Abellio Greater Anglia isn't the most reliable rail line, but I think there are some quite fast services into London from Colchester, if you have to commute.
The grammar schools of the King Edward Foundation in Birmingham are also superselective. Bits of Birmingham are also very attractive but obviously a rather less easy commute to London.
Of course, there's always QE