Well Steve, like others have stated/intimated the benefits are hard to find these days.
As per the FAQ section on the AUK website.
Can anybody ride Audax UK's events?
Yes.
Although technically, AUK does not 'run' events - these are run by clubs or individuals under AUK's supervision.
When a non-member enters an event, there is a small additional fee over and above the usual entry fee, which gives the rider 'temporary membership' of AUK for the duration of the event. This is necessary for insurance reasons.
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As an aside, there's not really such a thing as "non member", as anyone that enters an event is a member. Either by annual or five-year membership. Or event membership, for those that don't fall into either of the two former categories.
Then of course there are the true freeloaders, the CTC members. No need for any form of membership or on the day AUK membership.