Quote:
"At 11 am on September 3, 1939, when the British ultimatum expired and Britain declared war, the diplomats of the British Embassy gathered in the embassy's meeting room and stopped the clock. Ambassador Nevile Henderson and his staff immediately began closing the embassy down.
About 4 pm, the telephone lines were cut. German soldiers [EDIT] and Gestapo agents arrived to detain all British staff at the Berlin embassy and other staffers working at the nearby Hotel Adlon. The diplomats were then moved out of Berlin to a cushy arrest at the resort of Bad Nauheim, where final arrangements were made through Swiss diplomats for Germany and Britain to exchange their embassy staffs. The British were back in Britain on September 7, although most of their personal effects remained in a diplomatic limbo in Switzerland."
Source:
https://www.reddit.com/r/AskHistorians/comments/1qdwkb/its_the_3rd_of_september_1939_im_the_british/
It is to be assumed that reciprocal arrangements were made, as the German ambassador in London ended up spending most of his war in Poland, rather than being held in the UK.