Hi, regarding the schools you've mentioned:
City - probably the third most sought after school in London after Westminster & St Pauls. Very tough entry. If Belmont MH is 'more at DC's level' I'd not even bother with City.
Habadashers - roughly as hard to get into at City, especially in the last couple of years. Very pressured. Amazing campus!
St Albans - nearly as hard as Habs. Not renowned for good pastoral care, so I have been told. A lot of people apple to it along with Habs.
Forest, Bancrofts, St Edwards - I dont know much about these. Not as famous as any of the above. Forest appears more academic than Belmont though, going by league tables.
Highgate - about as difficult as St Albans to get in to, I believe. Stunning school!
University College School - slightly easier than Highgate to get into. All boys. Great facilities. Another really amazing school you should consider.
Belmont Mill Hill - they select pupils from just above average upwards. Best facilities and grounds that far in to London. Would certainly be a great choice for a sporty boy. Lovely school! Becoming very popular.
North Bridge House - i imagine entry is a bit like Mill Hill?? Not too sure what academics are like, but an awful lot of parents were very unhappy with the school a couple of years ago due to management issues. Plenty of Mumsnet threads on this topic!
Aldenham - lowest academic results of any of these schools, but an absolutely lovely place to go. Great campus and pastoral care apparently cannot be faulted. I have never heard a bad word about the school.
Hope that helps!