A life sentence is automatic for murder (in the UK). But a life sentence doesn’t mean life in prison. It means that the conviction will never be spent. Convicted murderers typically get a minimum sentence (whole life orders are exceedingly rare) and then it is up to the parole board when they are released. But if they commit any offences while on parole they can be recalled to prison immediately. Jon Venables is a recent example. Convicted as a child in 1993, released in 2001 then recalled (twice) for possession of CSAM.