From The Daily Express:
YANIS Varoufakis has urged Britain not to get caught up with EU negotiations in a stark warning to the Prime Minister.
Speaking to the Telegraph, the former finance minister of Greece said: “My advice to Theresa May is to avoid negotiation at all costs. If she doesn’t do that she will fall into the trap of [Greek PM] Alexis Tsipras, and it will end in capitulation.”
Said is said, he knows more about this than most people.
From ITV News:
[Jeremy] said a Labour government would approach the negotiations with "respect and sense".
"She (Mrs May) seems to be sending rather mixed messages. To start negotiations by threatening to walk away with no deal and set up a low tax economy on the shores of Europe is not a very sensible way of approaching people with whom half of our trade is done at the present time."
Correct, nobody should ever 'threaten' anybody, that implies that it is something you don't really want to do, leads to all sorts of confusion and might harm your own position*. You start off by saying what you actually intend to do, i.e. "set up a low tax economy on the shores of Europe". If the EU objects to this, they are perfectly entitled to say so and request politely that you do something else in exchange for concession X, Y or Z on their part.
* Some sick bastards kill people because they enjoy doing it. A kidnapper usually has no interest in actually killing the hostage. What he wants is the ransom money and to leave as little evidence as possible, not a corpse to hide and a murder charge hanging over his head.