What was the question for this implicit rationale, and corresponding answer?
Diplomatic Immunity allows for diplomats in other countries to not be tried for their transgressions. Countries that signed the Vienna Convention on Diplomatic Relations allow for Diplomatic Immunity. All UN member states besides Palau, The Solomon Islands, and South Sudan have signed the Vienna Convention on Diplomatic Relations treaty. The U.K. is one of the original UN member nations.
The answer: yes
The question: Can United States Secretary of State do crimes in U.K. without being arrested?