Isn't this dependent on what is being traded and the variance of the economies in the market?
For example if, say, the USA wanted to negotiate a trade deal with, say, an Eastern Asian economic bloc minus Japan and China, would they be that bothered to push for a deal with the larger bloc (with its larger market) if they can instead negotiate a trade deal with one of Japan or China, who individually may have a smaller market, but a more valuable market (in terms of wealth of consumer)?
During the referendum campaign there was 'expert opinion' for both Leave AND Remain, in terms of both Economic and Politics.
There was also disdain for said 'expert opinion' if it was the opposite of what one wanted to vote.
The time for engaging was prior to the vote and the arguments put forward by those who wanted a Remain vote were clearly not enough to convince the kaleidoscope of opinions (built up over many years, not just in the last 10 years) that in the end voted for leave.
Economics was not the only part of the reason.
It is now up to Parliament to decide how the UK leaves the EU; that is where any engagement at this point should to be done - and not only from an Economical point of view