At start, the town of Londinium isn't built as it was abandoned after the Romans left, while the county called Londinium in Roman (c_middlesex) is Noxgaga in the hands of the Anglo-Saxons. Therefore I don't see the concern around the city, as it is possible to rebuild London as a city similarly to how it was resettled historically.

London collapsed within a few decades of Roman rule ending in 410. It basically became merely a safe haven for local Britons to hide within the walls. When the Saxons refounded the city they did so outside of the perceived haunted Roman walls in what is now called Westminster.

After all, the dark age was even darker in the British Isles