Nick,
I agree that for a popular meeting it does make sense for members to indicate whether they are coming, though I would not book a place unless I was 100% certain that I could attend. That is why I waited until I did to attend.
I had not read the email, but I fully expected that the rules would have been the same as 2014 where the email said:
It will be free to all BAA members and affiliated society members if booked
in advance and for non-members the cost will be £5.00. If you turn up on
the day having not pre-booked the charge will be £5.00 to all.
It is reasonable to expect that it would be the same this year.
Tony.