Don't think it's unilaterally, if nothing else for that exact reason of countries permanently being in the process of exiting and wasting lots of time.
It's entirely possible to me that Britain tries, but the unanimous consent that's needed for the EU to grant that cancellation doesn't come through, resulting in Britain getting forced out, deal or no deal. They may then seek reentry.
It's entirely possible to me that Britain tries, but the unanimous consent that's needed for the EU to grant that cancellation doesn't come through, resulting in Britain getting forced out, deal or no deal. They may then seek reentry.