It doesn't have to be over 100 miles, and you do not have to be a permanent member. Take a look at the Audax UK official website, where there is a calendar of events. Many of them are 100Km, and there are a handful of 50Km ones thrown in too - along with some real beasts of 200km, 400km or even 600km. I do the 100km Windrush Winter Warm-up from near Cirencester every February. Brrrrr! It's always a cold one.This year the weather was warmer than usual , but we all ended up cycling with our pedals under water for over 100 yards in one flood. Always feels like a statement of intent at the start of another year's cycling.
I've also done the 110km 3 Counties Cloverleaf audax from near Evesham, and next week I'm going to be doing the "Silk Run" from Tewkesbury. In my (limited) experience, I agree that they are always friendly and well run events. By paying to enter, you will be insured and there'll be plenty of cheap tea and cakes. It is not a race, and it really doesn't matter whether you ride solo or as part of a group. I have enjoyed both.
If you enter one, then I really would advise you to sort out your route sheet how you like it before turning up, though. I always convert mine from km to miles, and measure out each stage separately so that I can reset my computer at 0.0 for the start of each stage. That way, if you go wrong (the instructions can be confusing at first) you will not have to do mental arithmetic all the way round to follow the distance instructions. I've just been spending about an hour changing my route sheet for next week, and I'll be photocopying and enlarging it for clarity before the event.