There's another chance to gain entry to RideLondon100 as part of British Cycling's Club Challenge. All you have to be is a member of Windrush and British Cycling.
If you're not currently a member this isn't an issue, but you'll need to have signed up I assume by the time the entries are allocated. 'Ride' Membership starts at £41 and if you find yourself spending a lot on cycling equipment the discounts you get could see you earning this back - it doesn't specify the requirement for a race membership or licence.
To summarise the below link, BC have space for 300 teams of 4. Teams must be made up of 4 men / women or 2 men and 2 women. It's not absolutely clear but I assume there's some kind of competition involved!