It is a questionable cause, as low UK-EU business optimism will not result in international sanctions. It is generally considered good etiquette to have cited something credible in the first instance, before blithely demanding proof of others.
You're making the outlandish claim that 'leaving the EU is like having sanctions' - the burden is entirely on you to 'prove it'.
Since most nations 'outside the EU' like Norway, Switzerland, or even Canada, the US, Australia are not comparable to 'being sanctioned' I suggest you're going to have a hard time making your case.
We aren't being given even the prospect of a trading relationship like Canada, Switzerland or Norway as they would require flexibility on one or other of the UK's immovable points. That's been clearly shown for years now, ever since Michel Barnier put out his very simple slide showing the trading relationship options.
Both the USA and Australia already have trade deals with the EU. The UK does not, and will not for probably years. International negotiation isn't a simple weekend thing like the Tory party keep trying to persuade the ever-gullible UK electorate.
So that's potentially years with nothing. WTO rules. Effectively self-sanctioning to a position far worse than now, and placing a severe cap on potential. Businesses with sense, or heavy EU dependency will leave, in droves. I would expect Toyota and possibly BMW to be probable early exits.
We could, I suppose, get a miracle of a fast agreement, but 26 nations have power of veto, and GB's negotiating team has not shown anything resembling flexibility or any apparent clue how negotiating works.
Trading on WTO terms is not like 'having sanctions'.
The EU and UK will continue to trade on some kind of terms on most goods and services until a deal is worked out. It will take longer than one year, but the UK-imposed 'deadline' is highly rational, and good for both sides, because nothing ever happens on the 'difficult issues' until both sides are up against a wall and are forced to 'show their cards'.
The 'one-year' milestone people should understand not as the point wherein negotiations end, but rather when the hard negotiation begins.
They will keep the status quo until that wrestling match is done.
In comparison to being in the single market, yes it is. At moment of EU exit we lose ALL trade arrangements as all we currently have are under the auspices of the EU. That would, for a couple of years until treaties were getting signed, put us in a worse position than every WTO member and observer.
In the first instance NO countries trade solely on WTO default terms. None. Every country has better than WTO arrangements with someone, even North Korea who aren't even in the WTO. Perhaps their local neighbours and major trade partners, even if they don't necessarily have a deal with the EU (or China or USA).
On WTO terms we'd need to negotiate tariff agreements with the WTO on potentially hundreds of categories of products which is going to take at least as long as an EU-UK treaty. As frequently reported, goods and services to and from the EU will be subject to import duties. For any of the numerous industries where that may happen multiple times, e.g. car manufacture, the effect would be disastrous.
Second we'll lose all trade agreements with other nations that we have had as a result of being within the EU, e.g. with Canada, the US, South Korea, etc, etc. So we'll have no improved relationship with anyone at all. It remains to be seen if claims of interim arrangements are anything other than empty rhetoric, so we still potentially face that cliff edge in Dec 2020.
Third we'd lose the unrestricted single market right to provide services (including financial) across the EU without discrimination and fall back to only WTO terms. That's likely to completely change viability of numerous successful UK businesses. Including some of the largest.
This is not just hyperbolic, it's false.