The year is 1952, as given in the first scenes of the show. The king died in February. It's obvious from how people dress outdoors that it's summer. Judge Peel presents himself to Tuppence as KC (King's Counsel). He should have said QC (Queen's Counsel) as Princess Elizabeth had already succeeded her father at his death, being Queen Elizabeth.
Tommy's lost wedding ring features in the plot; however, British men did not routinely wear wedding rings in the 1950s.