Let's assume the UK PM has said the UK will leave the EU. Actually we don't need to assume that.
Not one, nay, two UK PMs have said so.
Let's assume some other governments actually believe her.
Let's then assume they would like to have an FTA with the UK in the putative future post EU environment.
Is that really so mystifying and incredible to anyone?
No, but maybe they'd like they'd like to know first what the UK wants in the first place. Boris went AWOL after the referendum, and Davis now has a pie-in-the-sky wishlist which is simply not realistic.
Perhaps they are just not as scared of the EU as you both are, and don't believe that the relative bargaining power is so extremely lopsided as you seem to.
I mean, it's possible you are exaggerating the situation because you don't think the UK leaving the EU was a good idea. That's a pretty common response. But that doesn't make it correct, or sensible.
So what bargaining chip(s) does the UK have? The UK has a population of 65m and a GDP of $2.7t. The rest of the EU has a population of 440m and a GDP of $16.5t. The pound has dropped like a brick. Major financial institutions, which is the most important sector in the UK economy, have already announced plans they're leaving the sinking ship. I don't see much in the way of bargaining power.
And besides economic reasons, there's a very good political reason to make it difficult on the UK to leave. The UK has in the last 43 years continuously been bitching and moaning that they paid too much, and that they wanted an exception for this and for that etc, and the EU has indulged them in that, and it's still not enough. Then it should be abundantly clear that leaving the EU comes with a hefty price tag.
And that is not comparable to Norway or Switzerland, which never became a member in the first place.
Just because some EU officials have issued a (pretty unenforceable) edict of "Don't mention Brexit before Article 50", that does not constrain informal extra-EU negotiating. Does it?
It is foolish not to pursue initial negotiations for post-Brexit trade.
No it isn't. There are very good reasons not to start even informal negotiations.
1) There's nothing to talk about a FTA as long as the UK is still a "normal" EU member.
2) Starting informal talks while the UK PM drags out invoking article 50 makes a mockery of the two year deadline mentioned in the Lisbon Treaty.
Begin this year, Cameron negotiated with the EU some extra perks before the referendum. And he promised to take the UK out of the EU if Leave won. Leave won, and instead of invoking Art 50, Cameron resigned with the prospect that his successor would invoke it right after the start of their tenure in September. Now his successor is there earlier than expected, and she says she will likely invoke it end of the year. This is all a grand delay tactic. If the other EU countries start informally negotiating before Art 50 is formally invoked, there's no reason at all for the UK to formally invoke it timely and they can drag the whole process on and on, and in fact turn this in another round of "can we get some extra perks" negotiations.