@pretensesoup The author addresses this in the text: after the dead of Pitt the Younger and the independence of Haiti, the whole world stopped abolitionist work--the fear was that too many free Blacks would do what Haitians had done and overthrow white colonizer powers.
The tide turned again, and soon, but in 1806, yes, there was backlash in the UK--the backlash in the US has never stopped.