Hi Betty Girl,
I'm cetainly no expert on EU legislation however looking at the
EASA FAQ's it looks like from the answers from "Does EASA deal with Flight crew Licensing and Flight Time Limitations" and "Why is an EU agency needed to look after aviation safety" it looks like where the JAA was a body to suggest rules and common practice to be adopted by member states EASA is a centralised body to enforce legislation for EU member states.
So in answer to your question I don't believe we or the CAA could opt out.