Opinion is now something we can negotiate with? That’s a new one. I don’t think opinion has made any difference to the negotiations. David Davis doing eff all for the first two years, more like it.
Of course opinion is part of negotiation, it always is.
If you relied only on facts then negotiation would hardly be necessary.
Every transaction is based on opinion, from what phone you buy, to how much you pay for your house, to whether you go to war or not.