Why are many White British girls dark-haired rather than blonde-haired?

I somehow find it interesting. I didn't do biology and so don't really understand the origin of these traits.

Compared to the world at large we are very light haired on average. We just don't have so many platinum blonde types as, say, Sweden.
I like to consider myself flaxen blonde

I somehow find it interesting. I didn't do biology and so don't really understand the origin of these traits.

Because blonde hair is determined by recessive genetic allele, which means it takes the same gene from both parents to determine such hair colour. If at least one of the parents have dark hair allele in genotype, the child's hair will be dark.
That's mandatory primary school knowledge where I grew up.