It all boils down to the one dream the UK has: "Nothing bad will happen, because we are big and they depend on us."
That's the only straw you have. To put this into scale: The EU is eye to eye with the US. Not the UK. The UK is a small partner for the EU. So when you talk about "The UK being the 5th largest economy", that's a bit of a distortion. Let's look at the numbers, shall we?
1 United States 18,558,130
— European Union[n 1][8] 16,477,211
2 China 11,383,030
3 Japan 4,412,600
4 Germany 3,467,780
5 United Kingdom 2,760,960
https://en.wikipedia.org/wiki/List_o..._GDP_(nominal)
So, the UK's economy is roughly 6 times smaller than that of the EU. Before we're interested in the UK, we're dealing with the US (who we are about to slap in the face for the way they negotiate TTIP), China (who we extort wherever we can and block trade whenever it suits us), Japan (compare unit costs of consoles between Japan and the EU for a fun comparison). All of those are more important to us than the UK. If they don't get favourable deals where the EU let's them buttrape us, why do you think the UK would get such a deal? Because we're old chums? Don't be ridiculous, that ship has sailed. You wanted to make this about business, well, it's about business. If the EU wants polish workers to have access to the UK labour market in exchange for your access to the free market, that's a pill you'll be able to swallow.
Put this into perspective, you have about 1 million Poles in the UK. That's a very, very generous estimate about who's in your labour market, as I'm taking into account just polish people, not polish working people. And to get rid of them, you want to sacrifice access to the free market, a thing that you repeatedly acknowledge is essential to your economy. This is a ridiculous discussion by those merits alone, but let's not stop there...
It's not us forbidding you to access the EU market. It's us talking about free movement of labour. How exactly do you think you're going to do financial services in the EU if you can't send your workers to the EU to work in the banks that are going to be relocated from London to the new EU's financial centres? That's a big part of your economy is it not? Did you think you get to keep the financial service privileges the EU granted you? Seriously? A foreign nation having access to internal regulatory mechanics of the EU? It's crazy enough that you could meddle in Eurozone affairs as it was, but at least you were a EU member with a common interest. Now you're hellbent on fleeing the EU, don't count on the EU giving you
that kind of access to do damage to the EU economy.