It is this Sunday, only 26 miles and they do have a late entry desk. It's meant to be sponsored but you can turn up and just pay the fee of £10 to go round (£2 for 12-16 year olds - bring your son and his mates?). I have entered before with no sponsorship; they're just happy to get people on the course. Obviously a donation is always welcome.
I think there's a minimum age of 12. There's usually one that goes round Sheerness once a year which is open to any age (as there's little or no traffic) for the British Heart Foundation but this has been cancelled by the MOD (or whoever the defence contractor is) as they own most of the Island. That's usually a nice ride.
Alternatively just go out for a nice ride and save yourself the money.