No, you don't need to be a member. As it happens I've just seen a post on the Audax UK FB page that they are rolling out a new online entry system, so maybe there's been some issues with that? Might be worth trying again tomorrow.
I did read that you need to register with Audax UK now to sign up for an event online - it's free to just register, you don't need to become a member. There is something like an additional £2 or so charge on all events if you're not a member though.