Complex sentence: This is not an actual room (although the Bar and the adjacent Old Court Lounge constitute a space that might be called the JCR): the phrase refers to the whole body of undergraduate residents of the Hall.
Split into simpler sentences: This is not an actual room, although the Bar and the adjacent Old Court Lounge constitute a space that might be called the JCR. But the phrase refers to the whole body of undergraduate residents of the Hall.