The Arnold Hague database of BC convoys: There were four convoy series which carried the BC prefix.
The first was from Bristol Channel to Loire/Quiberon Bay from September 1939 to June 1940. This is a confusing series with F and S suffixes and the segment both ways bearing the same number. (Hague denoted this series by affixing '/1', F or S to the outward convoy and R to the returning one)
The second was a one off series numbered A, B and C from Freetown to Liverpool, Durban to Capetown, and Capetown to Freetown, respectively, in June and July 1940.
The third was a single U.S.N. convoy of BC 100 departing Charlestown, South Carolina on 27 Jan 42 and arriving at Bora Bora on 17 February 42.
Finally, the fourth series ran from Beira to Durban from July 1943 to August 1944.