It's a global issue, not just a UK one.
I'm not sure it's a bad thing, I don't think humans are particularly good for the planet.
And I'm definately not convinced that finances are the major factor. Russian women get the equivalent of 2 years salary as a bonus when they have a second or subsequent child and they've got one of the lowest birth rates in the entire world.
It just seems that, when women have choice, they choose not to. They want to do other things instead.