And? Trade does not necessarily require trade agreements ... it would also be pretty easy for the UK to just mention to 3rd parties: "hey let's start off with our existing trade agreement on day one, just find/replace 'EU' with 'UK', then negotiate better terms for us both immediately"
You don't understand how any of this works, do you?
The size of the EU gives it more bargaining power overall than its individual member states. That means the EU can and has negotiated trade agreements that are better for its members than any of them could obtain individually. That includes the UK.
Approximately 50% of the UK's exports go to EU countries (
source - 54.3% to European countries, but Switzerland at 4.6% is not an EU member.) Nearly 15% goes to the USA, more than 6% to China and Hong Kong.
That's a total of 70% of all the UK's exports going to trading blocs or countries that the UK needs far more than they need it - so it makes ZERO financial sense for them to allow the UK to continue trading at the current EU rate. They will lose a little in the short term if they play hardball by refusing UK imports unless/until the UK renegotiates its trade agreements, but the UK will lose a hell of a lot more.
I can tell you that those countries and blocs are already drawing up new trade agreements in anticipation of Brexit, and that they will drop them on the UK like a ton of bricks the day after it leaves the EU, and they won't take no for an answer. In fact I would be entirely unsurprised if the EU (which is almost certainly going to tell the UK to get f**ked out of principle) isn't negotiating with the UK's other large trading partners to ensure a united front to that end.
Putting my tinfoil hat on, it's even possible that these trade partners make an agreement to flat out refuse UK exports regardless of what the UK is willing to negotiate, in order to drive the UK economy into depression and drop the value of the sterling even further.
Hoo boy, it's gonna be fun times when the UK tries to get back into the EU a decade or so down the line, because Brussels is gonna be holding all the cards and it sure as hell won't let the UK forget it was the party that left.