We may not need to initiate article 50 of the Lisbon treaty.
We can ask the EU to write a new article; one that defines associate membership (on agreed terms) and that moves the UK into that status. This will include such items as reconfirming passport status on financial operations.
Being the EU's largest internal customer gives us the ability to negotiate this sort of thing.
That could mean years of non-clarity all the while conspiracies of Eu not allowing Britain to leave would run wild. Look at these threads - over half of the posters believe that to be the case.