As for the UK economy, on the macro-economic scale (external business) obviously when you are part of a large trading block you surf the wave of the clout that a block of 28 nations has when it does its deals, but you can also do your own, which people in the UK do not understand apparently, because all the “benefits” the Brexiteers crow about, the UK already has, only the people are too stupid or misinformed to know it. On the macro-economic scale with micro-economic effects (internal business) if you are part of a large trading block with which you do half your trade for free (paying the administrative costs as in any business) when you leave the block, lose the benefit of collectiveness (called synergy) and start paying to do trade, then you pay more for what you import (meaning higher prices, meaning inflation); your exports, being more expensive, are therefore less attractive so you sell fewer goods, which means you make less money. It means less revenue in taxation for the kitty, which means fewer resources available for the delivery of public services. Less money for schools and hospitals.
On a purely micro-economic scale, this is all translated into already tight margins squeezed to the point of no return, which means factories closing, Small and Medium-sized Enterprises failing, which means massive unemployment, which means within a few years technologies will have moved on making the unemployed chronically unemployable, which means higher costs in social benefits payments, which means fewer resources for public services. Less money for schools and hospitals.