You don't need to direct me anywhere as I'm not in disagreement with you.I could direct you to all the posts that have covered that point, but simply put, diplomatic privilege is granted by the Receiving State (the UK) and extends from the diplomatic officer to his or her family and accompanying servants. This allows diplomats all over the world - sometimes in quite hostile environments - to go about their business knowing that their accompanying families are safe. Diplomatic immunity (per se) is only valid for the duration of their posting and only in the country of accreditation.
The point i was trying to make is that her job may involve activities that, if discovered, could prove highly embarrassing to the UK government. With Diplomatic Immunity, publicity could be avoided either by preventing a court appearance or by declaration of persona non grata and a quick flight home.
The plan breaks down when the "diplomat" flees before being declared PNG as there's no element of punishment to appease the public.