Act of God - S1-E4
Corrected entry: Elizabeth worries that if she dismisses the Prime Minister she will be violating the Constitution. Tommy Lascelles later observes that her father George VI "never would violate the Constitution," Great Britain has no written constitution, only convention and custom.
Correction: Not "only convention and custom" - a lot of laws! Britain's constitution absolutely exists, but is "uncodified", made up of Acts of Parliament, court judgments and, yes, conventions, rather than a single document. Doesn't mean there isn't one.