UM - I used to teach a module on International Trade and I knew from this International trade was increasing as a trend all around the World year after year following WW2. An example was China joining the World Trade Organisation and opening its markets up more to non Chinese goods. Posters on here I am sure would agree its become much more common over the last 30 years to see Chinese made goods in the UK. In the last 3 years it may have changed though with regards trade.
I personally would accept the UK in the Single Market, there are other major trade blocs around the World that work as Single economc markets, but that is different to the EU (for me). I fully understand the EU compromises of different Member States. However as you know doubt know there is also central functions to the EU, such as Euro currency, European Bank managing the economies of the Euro Zone, EU Commission (law makers and sometimes enforcers) EU Parliament (debating forum). The EU appears to becoming a tighter Federal syle collection of Member States. This is a observation not a judgemental statement.
Not sure where your last sentence has come from "You seem fine with national protectionism, but not international" My outlook could be a little niave, but as a general rule I believe in free markets full stop. That means no tariffs, quotas, non tariff barriers (where common standards are met) for the UK and other nations. I do realise there can be difficulties where Industries are receiving state subsidies (such as cheap energy, farming subidies) and use these sell at below cost. Back in 2015 Chinese Steel was being sold at below the cost ot raw materials with the support of the Chinese Government (despite China being in the WTO) which made life impossible in many markets for UK producers who were not getting subsisided by the UK Government at the time. A Single Market is a a bit of a compromise it doesn't mean full free trade on an international scale, so that is not consistent with my outlook, but a compromise and a bit of recognising the World we live in. We all are always learning too.