Just learned that there are more giant redwoods in Great Britain (~ 500,000) than in California (~80,000). It seems the Victorians really liked them and planted a lot of them. Now they are starting to outgrow every other tree in Britain, but they are still juveniles and not yet self-seeding:
Then I wondered: where else could there be sequoyas? So I found this map for Spain (article in Spanish, with images):
https://www.arbolesconhistoria.com/2021/09/23/mapa-secuoyas-de-espana/
They write there are at least 150 redwoods in Spain and probably many more small ones. And this site claims
there is a whole forest of redwoods in Cantabria, more than 800 trees.