Thats sounds like any number of high-density buildings in London. Funnily enough, installing one uplink and switch/router into the basement to then serve dozens/hundreds of tenants in a building is really easy money, compared to cabling up an equivalent number of homes - and of course it's all baked into the offer (whether free, or what looks like a good price for gig), it's a feature of the high-spec rental package, the building owner gets it at a fraction of the cost and the ISP gets a captive market, because the landlord won't allow a dozen other companies to bring in lines, run through all their ducts, punch holes through structure, etc. Look at the likes of hyperoptic.
"Londoners can't get more than 4Mb" is misdirection. Plenty/most londoners can get much better, plenty of rather rural people can too. But some unlucky londoners and more unlucky rural folk are behind the curve. It's the londoners I have sympathy for, because they're living somewhere you expect (and pay/rent to be close to) the latest facilities.