BHP and BBL are the ADR's for the BHP Billiton Group of Dual Listed Companies that operate as if merged, consolidate their assets and earnings, share the same employees, assets, officers and directors. BHP is the ADR for two BHP.AX shares, the Australian based half of the DLC's, BBL is the ADR for two BLT.L shares, the UK based half.
The BHP group keeps their combined books in US$, and declares their dividend in US$. Both BHP and BBL get the same vote and dividend, but for some unknown reason BBL trades at a discount to BHP.
All explained here on the BHP Billiton website under the Investors/shareholder info tab Q&A.