I don’t think you would be wholly surprised to learn that more BSFA members nominate novels for the BSFA award than any other category. For most multi-category awards, novels are the attention-getting category. They’re the units (other than film) in which the most socially-visible science fiction is advertised and sold.
This year, 43 BSFA members nominated a total of 60 books for the award shortlist. This is a smidgin down from last year when 47 members nominated for the category, but despite the four additional people last year, they nominated exactly the same number of books, 60, for the previous year’s shortlist! Both years were up from the awards given out in 2009, when only 26 different novels were nominated.
The BSFA currently has about 600 members, which means that about 7% of all members nominated one or more novels for the award. A gratuitously higher statistic is to say that one novel was nominated for every ten members, on average. Clearly, there’s room for growth – but equally, there are plenty of reasons to join the BSFA which have nothing to do with being able to nominate for the award. Keep in mind too that this is just the nomination phase I’m discussing here: plenty more people will be voting on the final ballot!
One advantage of Easter – and thus Eastercon – being later than usual is that, if you haven’t already, you have a little more time in which to read the five novels shortlisted from that list of sixty nominations before the ballot is due.
vector-bsfa.com 
Not sure that link is working – though I can find the BSFA shortlists.
Thanks for catching that. I’ve fixed the link.