We have the rules for the Southern Champs here: http://www.southernxc.net/round-4.html
To be eligible to be crowned Southern Champion you need to hold a BC licence with Southern, Central or South East as the region on the card. You don't need a licence to race for the series but you do if you want to be an area Champion. Anyone else from outside the region is welcome to ride our race and can still take the trophy for the on the day series result and cash prize for elite but will not be eligible for the Southern Champion title and medal even if they were born in area which doesn't count. Its all too hard to prove and check, easiest thing for us is to check the region on race licences.
SAMS started the regional Champs nearly 20 years ago and Southern Champs used to be from Bristol Channel up to the Wash and included South West and Eastern regions but as they have started their own regional Champs we have dropped them areas.
See also : http://www.britishcycling.org.uk/member