The Kraken Wakes is a science fiction novel by John Wyndham, first published in 1953.
Towards the end of the story, the sea levels are rising and London is beginning to flood.
"When the breakthrough came, it occurred in a dozen places on the north bank almost simultaneously. Among the spurting jets a bag or two would begin to shift, then, suddenly, came a collapse, and a gap several yards wide through which the water poured as if over a weir. From where we stood on top of an EBC van parked on Vauxhall Bridge we were able to see three separate rivers of muddy water pouring into the streets of Westminster, flling basements and cellars as they went, and presently merging into one flood.
(...)
From that point on, the thing got thick and fast. On the south bank water was breaking into the streets of Lambeth, Southwark and Bermondsey in a number of places. Up river it was seriously flooding Chiswick; down river Limehouse was getting it badly, and more places kept on reporting breaks until we lost track of them."
Most people are evacuating to higher ground, but the protagonists volunteer to remain behind due to their role as journalists working for the 'English Broadcasting Company' - fictional rivals of the BBC.
"Several weeks passed before we got wind of the fact that EBC had leased the top two floors of a large department-store near Marble Arch, and were working all-out to have them converted into as near a self-supporting station as was possible. 'I should have thought,' said Phyllis, when we acquired this information, "that somewhere higher, like Hampstead or Highgate, would have been better.'
'Neither of them is quite London,' I pointed out. 'Besides, EBC probably gets it for a nominal rent for announcing each time: "This is the EBC calling the world from Selvedge's." Goodwill advertising during the interlude of emergency.'
'Just as if the water would go away one day,' she said.
'Even if they don't think so, they lose nothing by letting EBC have it,' I pointed out. By that time we were becoming highly level-conscious, and I looked the place up on the map. The seventy-five foot contour line ran down the street on the building's western side. How does that compare with the arch-rival?" wondered Phyllis, running her finger across the map. Broadcasting House appeared to be very slightly better off. About eight-five feet above mean sea-level, we judged. 'H'm,' she said. 'Well, if there is any calculation behind our being on the top floors, they'll be having to do a lot of moving upstairs, too. Gosh,' she added, glancing over to the left of the map. 'Look at their television-studios! Right down on the twenty-five foot level. There'll be a lot of helter-skelter back to Ally-Pally, I should think.'
There isn't exactly a large department-store near Marble Arch called 'Selvedge's', but I think this is the building that Wyndham was alluding to 
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This cache is part of my ‘Fictional London’ series – check this list to see them all (more to come).
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