Membership of the FRA is not about just attending events.
What you need to be a member for is,

The ability to post on this forum (it costs)
The regulated set of robot build rules,event safety guidelines throughout the country. (it costs)Insurance.

Imagine five different event organisers with five different sets of build rules, with five different sets of safety standards.

Adrian the choice is yours but you certainly do not pay FRA membership to attend events.
The event organisers prefer you to have FRA membership as it gives them extra insurance and piece of mind.The FRA would not entertain claims from non members for use of our rules and regulations.A mixture members and non members at an event could be potentially a legal minefield