Let's see where the NSPCC stands here
What the law says
The law does not set a minimum age at which children can be left alone. However, it is an offence to leave a child alone when doing so puts him or her at risk.
Pretty much sums it up.
Child put at risk, offence is committed.
If a child is taken while the parents are away, then that is solid evidence that the child was put at risk - by the parents who were responsible for the child's safekeeping.
Should at least be presented to a jury.