Even if that weren't the case, I think it'd theoretically be possible for EU and UK to have an exit agreement retaining full cooperation, the UK leaving and immediately applying for membership again. That process could be quick, if everyone (which in this case would involve all member states) agrees about it... but could also be a point where some governments might try to strip the UK of privileges from it's current membership.
Maybe it can be done unilaterally too.
Even if that weren't the case, I think it'd theoretically be possible for EU and UK to have an exit agreement retaining full cooperation, the UK leaving and immediately applying for membership again. That process could be quick, if everyone (which in this case would involve all member states) agrees about it... but could also be a point where some governments might try to strip the UK of privileges from it's current membership.