There needs to be a neutral word describing the set of all people without offspring (regardless of cause or choice) and 'childless' is it - it (alone) doesn't apply a value judgment.
I understand the use of a different word - childfree - to describe not wishing to have children, but an insistence on its use in all cases introduces the issue of 'why' into places it's not relevant... like these ONS numbers.
It's also clumsy to group all the 'others' (the 'non-childfree'?) into a single group too. There are many different reasons and - if anything - it is these groups that lack neat descriptors.