The basic problem dear THM is that an awful lot of the UK, and indeed a lot of parliament and the government, don’t really want to negotiate because they don’t want to leave.
Now you can say there was a vote, or we’re leaving, until you’re blue in the face, but it doesn’t change the fact that we, as a nation, are at best halfhearted about this simply because we are indeed half hearted about it.
For a successful Brexit a consensus had to be built that supported the result of the vote. Nobody has shown much interest in that.
Or in other words, the vote was the start of the UK deciding what to do, and too many people have treated it as the end of the decision.