This is exactly what BIBOA is really about. It's a members' club, run by the members, for the members.
BIBOA organises whatever events anyone decides to organise, and offers a useful ready made framework for doing this. If people with small boats don't join, or join and don't organise anything, then it will continue to cater exclusively for larger boats.
With an increasing number of new members with smaller boats, I expect to see more and more small boat events coming up. It is the British Inflatable
Boat Owners Association after all!