They didn’t though. Nobody forced them to leave without a reasonable deal, or a reasonable bureaucracy set up to handle it.
They did that all by themselves.
I think they could have gone for a deal similar to the other non-EU but still kind of EU countries like Norway if they wanted. Because of pragmatism, but that’s not what the Brexiteers wanted.
Britain couldn't get a deal then vote for it, as the EU would then hold all the cards negotiating. I.e it will be have offered Britain nothing in order to stop Brexit.
Likewise if Britain had set up a bureaucracy to deal with Brexit before it was voted for there would have been a massive backlash about the waste of tax payer money on an uncertain event.
They did that all by themselves.
I think they could have gone for a deal similar to the other non-EU but still kind of EU countries like Norway if they wanted. Because of pragmatism, but that’s not what the Brexiteers wanted.