You have absolutely no idea what trade deals we will or will not be able to negotiate if we leave the customs union. But you assume the worst case scenario and then start stating it as fact.
You assume reciprocal tariffs. You then went on to mention how we could have tariff-free electrical goods exporting to USA without mentioning that your reciprocal tariffs would mean US electrical goods (of which we import more than export) coming cheaply into the UK and undercutting our home markets.
And in trade, the likes of USA, Japan and China are way more important to our trade than we are to theirs. They have all the leverage.
I don't assume the worst scenario, I expect the
likely scenario. That is: trade deals are hammered out over many years, involve all kinds of compromises and concessions, and generally the biggest dog gets the best bit of meat at the end of it. While the US is our biggest export market, we are way down their list of importance