The problem with the remain voters thinking it will be difficult to negotiate a trade deal post brexit is that they are projecting their own sense of weakness onto the negotiating team.
This nation, for all her faults and disgusting history, survived perfectly well from the middle ages until the 1970's without needing Luxembourg's permission to fish her waters, and trade with whomever she chose to, in which ever manner suited both parties.
The post brexit team will simply say, to Germany, you want to sell your cars and your engineering to the UK? No problem, speak to your partners about this trade deal, drop the tariffs and we'll do the same. It'll be just like before only this time we ain't starving our public services of cash to pay for the privilege of buying German cars or French wine.
There is no confidence in our nation on the remain side. We are the 5th biggest economy in the world, maybe the 4th shortly, don't tell me we couldn't survive. And thrive.