When I was looking for my AN kids I looked everywhere, but was most successful on the facebook groups and found a few breeders. One didn't have enough girls but another did and I got them 2 weeks ago, had to travel to Lincoln but was worth it.
Try the groups, Goats for sale & wanted UK and Pedigree Goats for sale, wanted and stud. Also Livestock for sale in Norfolk.
I struggled to find ANs and got the impression most of them kidded in spring, so it may be all available kids have found homes by now, but I could easily be wrong. Whereabouts in the South East are you? I'm Cambridgeshire.