The Child Support Agency (or CSA)
From the gov.uk page on CSA:
Child maintenance is financial support towards your child’s everyday living costs when you’ve separated from the other parent.
You and your ex-partner can arrange child maintenance yourself if you can agree. This is called a ‘family-based arrangement’.
A family-based arrangement is a private way to sort out child maintenance. Parents arrange everything themselves and no-one else has to be involved.