This act outlawed slavery in England as well as all of its colonies. It was in fact replaced over the next five years by an apprenticeship system that allowed slave owners to retain their property and have slaves work for no compensation. These abuses led to riots in the islands in 1839. Rhys cites this timeline in Wide Sargasso Sea, knowing it doesn't align with the timeline in Jane Eyre.  Rhys finds it necessary to insert it in order to allow her  narrative to align with the events of the times she describes, namely the destruction of Bertha's childhood home.