The South of England Athletic Association (SEAA) have published their competition prospectus for 2021.  This provides information about the Southern Championships, including the dates of the senior championships, and the various age group and combined events championships.  You can read a copy here.  The prospectus doesn't seem to tell you how to enter, but you can mark the dates in your diary, and then keep your eye on the SEAA website, which will likely provide entry information in due course.