A social barrier
The "poor" in Kent are not swilling around in grammars: many are in those under-performing schools. Ambitious families here buy a house in the catchment area of a top primary to ensure success in the 11-plus. Many children are also sent to private schools or get private coaching to pass it.
The system of first-preference admissions does allow access to good non-selective schools for those parents who genuinely want their children to go to them. Equal preference in Kent would simply mean grammar contenders having two choices: the grammar or, in the event of failure, the next best school - thereby displacing families who wanted no part in selection. Surely, if parents wish to retain the grammar secondary modern divide they must accept both sides of the coin?
No one wants a "sink" school but nearly everyone would be happy with a thriving comprehensive. How much easier it would be to resolve admissions anxiety by scrapping the 11-plus and concentrating on making every school a good school.