This bod was a bit inclined to tell not-very-likely-sounding tales of alleged past doings and experiences of his. Pretty well everything I had heard about the way things were in divided Berlin, caused me then to suspect that the above was an instance of this. However; I do get an inkling that over the decades, everyday doings re the "oppositional" sectors of the city became a bit more relaxed, in some ways -- feel that just possibly, this thing about "British Army day-trips" might be true, however improbable-seeming. Would be interested in thoughts from folk knowledgeable about such matters.
Okay, so how it worked in practice:
The Allies had freedom of movement throughout Berlin, as the city's civilian authorities derived their authority from the occupying four powers. The city was officially occupied between 1945-1991, hence why West Berlin wasn't an integral part of West Germany. The Soviet Union never formally ended this agreement, as they also wanted to maintain access to West Berlin. They threatened to do it a few times, but from a purely military point of view, East Berlin was part of the wider occupied Berlin. Checkpoint Charlie therefore came into being as the agreed checkpoint for Western Allied forces going between the West Berlin sectors and the East Berlin sector, along with Checkpoint Alpha in Helmstedt and Checkpoint Bravo in Dreilinden.
So, on a practical level, what happened was that military forces could move freely between West Berlin and East Berlin. If an American officer wanted to catch a theatre show in East Berlin, they could, just as a Soviet officer could (in theory - in practice, it wasn't typical) go see a theatre show in West Berlin. What was supposed to happen was that Allied forces would only identify themselves to Allied military personnel if they weren't in uniform, which is what caused this -
https://en.wikipedia.org/wiki/Berlin_Crisis_of_1961 - as the Americans refused to identify themselves to East German border guards, who had legally no right to control military traffic in Berlin.
The British on the other hand were pretty pragmatic - they would wave their military ID cards at the East Germans.
(note: the Western Allies made it somewhat clear before the building of the Berlin Wall that while they insisted on free access to West Berlin, they had little interest in what the Soviets did in East Berlin as long as their right of access wasn't denied to them.)
So, to sum up: Day-trips were pretty normal by members of the Western Allies. They weren't subject to East German immigration restrictions as long as they stayed within Berlin, and East Berlin offered *some* things at an incredibly good price, such as theatre/opera and strangely enough, sheet music. It would have been nothing out of the ordinary for a British soldier to go over to East Berlin when not on duty, although they were clearly instructed not to provoke the East German authorities.
.)
