Over organising something like this can make it much more complicated than it needs to be. It might be best if Brian just declares what time he'll be in the pub, and anyone who gets there can buy him a drink.
Given good weather it's a very straightforward crossing - just do it. If you're not happy with the forecast on the day, don't go.
Instead of trying to put a flotilla together, why not get together in twos or threes and all meet up when you get there? This will almost always be more satisfactory than trying to keep a big group together. In the time it takes to assemble a couple of dozen RIBs off Calshott you could be halfway across!
Find someone with a similar size boat and PM them and see what you can arrange . . .
If you're not comfortable doing your first cross channel in your own boat, then why not see if you can crew for someone more experienced?
Fingers crossed for the weather