The British could declare him persona non grata and order his
> appointment terminated.
They could order an appointment to the UN terminated? Even if it were something London-specific - say, permanent representative to the International Maritime Organization? I'm in over my head here, but I find it hard to believe that a host country can veto foreign diplomats appointed to international organizations. Could the US, for example, boot to Syria's UN ambassador out of Manhattan?
> He'd only have immunity in relation to his official UN duties.
But that doesn't seem like much of a hurdle as long as there's any London connection to be found.
-- "Hige sceal þe heardra, heorte þe cenre, mod sceal þe mare, þe ure mægen lytlað."