Very interesting video which just highlights the paucity of UK housing stock. To be frank, it looks like a slum! Poorly built by speculative builders in the late 19th century and poorly maintained over the past 100 years. My Danish wife is still horrified by British houses despite living here for decades. Take a look at Scandinavian housing to see how it's done properly.
That’s a white rot not a brown rot. There is a failed damp course. Why did you not take a sample and carry out a salts test which would prove ground water plus there is also penetrating rain water. The external plinth will be bridging any dpc in that wall.
