One aspect to Starmer's uttering that to me highlights a failing of current politicians: "I take full responsibility" just uttering the phrase means no consequences (for the speaker), no more discussion please, end of story, more on ...
It's become a meaningless phrase used to attempt to close down important issues. Starmer's judgement is a major issue and that he admits he failed means he should be putting in place procedures to prevent against poor judgement eg in the case of appointing senior diplomats that the Foreign Affairs Committee examine and approve (or can veto) proposed appointments. But no, Starmer wants "I take full responsibility" to be he keeps all those powers he's proved himself incapable of exercising properly.