While I'm here, do you remember that WTO Rules fall-back option? (you know, the "God help us" trading option assuming that we can't keep access to the Single Market AND kick Johnny Foreigner out)
Well, we'd even have to renegotiate entry to that club too - we can't just waltz in. And God-forbid if a WTO member had a long-standing UK thorn in its side, like ownership over some obscure South Atlantic islands, for example, and decided to be difficult about admitting us.