Yes that model would, however you forget both combined and doubled don't equal the same strength of UKs economic Power, Countries outside Europe smaller then UK operate perfectly fine with no issue(I live in one and without an actual trade union, Australia really looks like a struggling economy).
There is plenty of other trade unions UK can join without selling it's sovereignty, and with the amount of trade UK imports it would be a massive detriment to all those economy involved if they played with it.
Do you think Germany could afford to add a tariff to a country that buys 5 and a half Billion pounds worth of product from them a month?
It would massively hurt Germany's competitiveness in the UK market if you added a tariff to that and the Germans would Suffer.
it works Both ways, and considering UK runs a Several Hundred Billion pound trade deficit suggests it's impact to the economy's of those it bought from would be severely impacted if they where then to add tariffs to those goods.
Trade wise UK is actually in a position of power to negotiate a favourable trading deal if it did indeed left the EU, far greater then Switzerland or Norway could even hope to achieve.