Yeah i've found one, its the airport in the south, you have to do gordos? i think thats his name.. hes super hard to miss.. anywho you have to do his quest line which is just a few short quests.

Havent met anybody with a name like that! O___o

Still havent found anything to do with that guy - BUT...

London Bridge (NOT Tower Bridge! - it's next to the Ferryman's Dockhouse). WARNING! Settlers etc have a habit of falling through the bridge and drowning. DO NOT POPULATE WITH COMPANIONS!!!
Covent Garden
Lambeth Walk
and
A construction site south west of Lambeth Walk are all habitable.

Might be mission related (im leaving out spoilers).
So if you want to get building fairly quick, best i can suggest if following the main plot until you hit the ferryman's dockhouse.

Also I used the explosives box there as storage for a long time without issue. Its also where Churchill may end up when dismissed,

Edit: eventually found the "gordo" guy mentioned above. He's a distance south west of the swan and mitre. Not really on the trodden path.