I think it depends on the school - you could be lucky although the good ones will have wait lists. Helps if you meet some criteria (ie RC or CofE). best to get the schools info from the council, it will give you all the schools' details and you can check individually. I used to live in westminster and there is always a lot of movement of families in central london so you might find places suddenly become available
The council will treat you as a late application. You apply to them, NOT to the schools. There is no reason for them to wait until September before processing your application. The schools may be on holiday but the council isn't.
They will find you a place but it is unlikely to be at the school of your choice. It will probably be at one of the less popular schools in the area.
If you don't get a place for your older child in a school you like straightaway, be aware that he is eligible for a part-time or full-time funded nursery place (your choice) from Sept 2011 as an alternative to Reception.
If you prefer to wait for a place at a school you want, that could help with costs. His eligibility for that continues until he reaches compulsory education age in the term after his fifth birthday.
I've no idea how difficult it is to get a nursery place in the area, but that might be an option.
With voluntary aided schools you might need to apply to the school directly actually if you are arriving outside of the admissions period. the religious schools usually have their own application forms aswell as the council one. at least it was like that when I applied for my dcs
No, you apply to the council regardless of whether or not the school is voluntary aided. The last government changed the system this year so that you always apply to the council even if you arrive outside the normal admissions round.