Two things:
1st, states can and do regulate DMV issues differently. In some, you may have two valid licenses from different states, in others, they invalidate your old one when you get one in that state.
2nd, some states do REQUIRE that you are a permanate residence to obtian (and maintain) a valid drivers license.
I hate to say it, but if you have given up your home in CA and are only maintaining a home in the UK, you are no longer a resident to CA. Mailing address in CA or not.
I am lucky in that I have a home that I maintain in WA, and ALWAYS use it as my main address (thankfully, my family is there to gather post for me).
Sorry you are having the troubles...