I don't get why people don't get this.
Most government departments, financial services firms, law firms etc are based in London, because it's the capital city and they all want to, or need to, be near each other.
Because we're talking about big departments and firms, they have more jobs and end up having specific roles which are only done in those big departments or headquarters.
For example, my job is very niche and the majority of the roles are in big law firms, which are predominantly based in London. The job does exist in law firms outside of London, but in such a limited way that if I took the role of X in a law firm in Norwich, say, I'd be about the only person in the county doing that role, and there would be absolutely no chance of job progression. So career-wise, London is the best place to work.
Now there's nothing special per se about London, other than it is the capital city and it is where all these departments and firms are based. They could all decide tomorrow to move en masse to Chichester. But all that would happen is that property prices and all that in Chichester would go up, and you'd end up with the same problem there as you have in London.
It's really not rocket science. Big government and big business have specific roles.