Right after WW2 the RAF were using up a lot of wartime stocks so the main HE bombs used were 500 and 1000 lb MC of various Marks plus other assorted types such as the 20lb Frag bomb (based on the US M41). In the 1950s, after the WW2 stocks had been dropped or dumped, the 500 and 1000 lb MCs remained in service for carriage on Canberra, Victor, Vulcan, Valiant and other aircraft. When these aircraft had gone out of service as bombers both of these bomb designs were revised for high-speed external carriage (on Phantom, Harrier and Buccaneer aircraft) with twin suspension lugs. The 500 lb became the 540 lb while the 1000 lb Mks 6, 6*, 9, 11, 11* and 12 evolved into the Mk. 20; both the 540 lb and 1000 lb Mk. 20 are still in service today.
Hope this answers your question.
eodda