But that is because the 27 other countries are losing free trade with just one country whereas we are losing 27 partners. It's like half of what we sell goes to EU countries.
I don't know the full details (although I have just had a quick google at what Germany exports to us and it is 6.6% totalling 2.6% GDP which kind of backs up what I am about to say), but I'd imagine we are talking small percentage of each country is directly involved with us. So if the worst came to worst they'd be a couple of points down, but trade will not just stop between us completely as people will still want and just stomach the increases. So in Germany's case it might go down to say 3.3% and 1.3% of GDP meaning they are only down by a percent or so. If the banks decide to head out en masse to Frankfurt as they have indicated then that minus probably becomes a plus.
If we use the same sort of calculations for ease, immediately we are 25% down (4% GDP) + the decimated financial sector + the car plants that say they will leave + the fact that we services based economy means a lot can move elsewhere very easily. So the economy could have shrunk by 10% GDP in a relatively short period of time. Then you factor in the shock, investors and companies will look to see how things lie but because we have lost 25% of the market, it will lead to job losses and companies folding which puts more pressure on the state. This leads to people tightening their belts, which leads to more job losses and so on.
And if that wasn't enough we have a country attached to us that will want out of our political and economic union if we leave the the EU with a no deal. This further adds to the uncertainty which will effect the markets.
Hopefully that gives a taste why I think other countries although will take a hit, it is nothing like the one we will take. It is also why I think the EU wouldn't be interested in a giveaway deal to keep us in as they will be able to absorb the shock better and indeed look to benefit from it by taking some of our companies/industries that still want to be inside of the EU.
I will agree on your last part, hopefully we never have to find out.