The British empire had outlawed chattel slavery and had gone as far as interdicting the slave trade to South America, using the Royal Navy.  Could Southern slavery continued if they had become a "British puppet state?"

A "no" is supported by the fact one of the principal reasons the British Empire did not intervene was slavery.

A "yes" is supported by the fact the BE was acquiescent of the caste system of India.  From what I can gather, they didn't like it but they didn't like it enough to try to force its abandonment.  

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre