What a wonderful idea!
I read somewhere one of the weekend broadsheets that street parties used to be a spontaneous get together but especially in London there's problems with red tape and health and safety - try and contact the council (Lambeth I asssume?) and the police as early as you get a date - there was mention that it may cost for the street to have a party and to meet all the health and safety regulations - i hope this isn't the case as it seems ridiculous.
Good luck with the planning - if only my street was as sociable...yes I know I should organise one myself!!