I've never been on one - so I can't comment, but the Bath one wasn't fully booked up yesterday .... not sure what procedure there is to stop someone who isn't registered joining in... or how to prove that I have registered.
Did our first SkyRide today, 15 miles in Bexley, Kent. Registered on line last night with 4 places left, by the time we had finished it was full.
32 people booked, some who had registered failed to show, but others turned up on spec and took their places.
It looks like you have to book in advance, but there is the opportunity to cancel if necessary. All the popular ones in the south appear fully booked, but there's also a waiting list you can go on.
Have booked a few more, nice friendly ride and well marshalled.