It's endemic in Zone 1. The sole provider is BT, generally only adsl services, though VDSL is sometimes available. As an example, there is no residential BT or Virgin fibre available in Soho or Mayfair. There are small pockets near Bloomsbury and Marylebone, but that's about it. 4G is frequently a joke due to reception issues and interference and microwave is astonishingly expensive.
The reason is that this area was traditionally commercial, so if you want better-than-adsl internet you need to pay for a commercial link. None of the vendors want residential grade circuits eating into their highly lucrative commercial business.
We have dozens of residential customers who are stuck with either ~12mb ADSL on old dodgy phonelines (one recently refitted 20 apartment building has exactly six functional pairs on a 100pr bearer) or a minimum £400/month for a 25/100 leased line, which is barely "high speed".
Sure, if you can afford to buy a house in Mayfair, you should be able to pay that sort of rate, but most of the shoebox flats are done on the cheap so the internet provided is a phoneline if you are lucky. The best landlords bring in a proper leased line and split it among the tenants, but that is rare.