We keep our house at 17C/62F. I also have wood instruments, some quite expensive, for which I track humidity with a slight bit of obsession. I say “slight bit” because in the Seattle area the indoor humidity of our house rarely exceeds 50%, usually 35-45% in winter. That’s about perfect for wood instruments, and it rarely varies much, so no need to get too obsessive about it. And our house doesn’t have a mold problem.

Point is, depends on where one lives, I guess. It’s odd that you call out Britain, as that’s about the same latitude and same rainy winter weather as Seattle, but our humidity stays within a reasonable range.