Additional:

What’s to stop me becoming the registrar for .microsoft, .paypal, or .natwest.

The fact you need to present ICANN with $100,000 and a business plan explaining why you should be allowed it and how you will generate revenue… The council may be able to register .london and then sell domain names under that to local businesses, but you won’t. Hopefully they’ll notice that you don’t, in fact, work for Microsoft.