Right now, if you want to IPO a Germany company, your friendly Goldman Sachs banker in London can do the whole thing over email.
But from 00:01CET 01/01/21, that banker will need to move to the EU, establish residency, get approval to open GoldmansEU, open it (including getting the correct training and certification and moving a tonne of money there), setup to pay taxes and abide by local and EU regs, and then call the client back.
He'll sack his UK staff or try and take them. He'll stop paying UK taxes. He'll stop funding the UKs Museums and Operas and Lamboghini dealerships.
Financial services for the world's largest, most developed market has been a massive boon for the UK. Its funded everything else we've done.
Passporting was the core economic thing we wanted in a trade deal. Fishing and farming and manufacturing are all worth less than just banking. And we didn't get it.
But we can continue to sell some cod as long as we abide by Brussels decisions so I guess I need to retrain...