] By the time she wrote Mansfield Park, the anti-slave trade sentiment in Britain was so strong that the British negotiators at the Congress of Vienna had to include the abolition of the European slave trade as one of their requirements for the Congress.

Here's a link to something I posted earlier, about British public opinion and its effects on the negotiators at the Congress of Vienna. To quote just one sentence, the chief of the British mission (and Foreign Secretary at the time), Castlereagh, wrote: "The nation is bent upon this object... and the ministers must make it the basis of their policy."

With this sort of widespread strong public opinion, it's my view that the burden of proof lies on those who would argue that Jane Austen did not share the general sentiment, at least in 1814, when Mansfield Park came out and Emma was in the offing (these being the two novels which mention the slave trade). I'd argue that in 1814, Jane Austen, an intelligent, well-informed and humane woman with no financial interest at stake, would have no reason to go against the prevailing opinion in England at the time.