In a country with a low cost of living, one can live like a king for £1k a month. With £400k that's yearly withdrawal rate of 3%, which seems safe enough.
If the world doesn't go to hell, I will keep farming the excess back into working assets - most likely property, following the same formula of buying shitholes in nice areas and turning them into miniature palaces and renting them to banks and tech firms.
If the world does fall apart, we have everything we need here to be entirely self-sufficient. Land to grow food on, a river for power and water.
We're actually spending less than €500/mo on living expenses - more on hardware, tools, materials - which we do in Spain (as does everyone else in this neck of the woods), as it's a lot cheaper than Portugal for that kind of stuff, and the border is only 10km away. For the really esoteric stuff, there's Alibaba, if you don't mind waiting for the slow boat from China.