All rides that are officially validated by Audax UK are listed on the calendar - each listing provides a link to the event page and online entry form, simple as that. There are rides virtually every weekend, and midweek, from locations all over the UK. Below is a screenshot of just the rides of over 200km (a valid Audax ride can be as little as 50km) in the South East of England from May to July 2018.
Once you have entered and paid the entrance fee you can expect to be contacted by the organiser with a routesheet, GPS file and some information regarding the ride start, finish and control information. And that’s it, then just turn up at the start (preferably on time, or it could be a lonely day out) and enjoy riding your bike.