The other branches in the county are Ely, Fenland, Huntingdonshire and Peterborough (see East Anglia branches).
Your "local" branch is determined by where you live - if you live in the branch's area, you're on the HQ computer as being in the branch.
If for some reason you want to be registered as being in a different branch, then you need to contact HQ to ask to be transferred.
All CAMRA members are entitled to attend all CAMRA branch meetings, irrespective of branch affiliation. If you just want to come along (or even help out?), then just come!
In practice branch membership is only relevant for formal branch meetings such as the Annual General Meeting, at which only branch members can vote.