Jason
my club, in Essex, runs in three leagues. There is the Essex League which is the big one which tends to be of a better quality field (so I believe), the North Essex League which has clubs from Ipswich, Dunmow, Colchester and Chelmsford, and the Mid Essex League which is meant to be a bit better for people's first experiences of cross country.
I think its possible to enter all three as a non club member, but I could be wrong.