Some additional points
The phrase "Land of the Free" dates to 1814 with the poem, and later song, "The Star Spangled Banner" so references to it in regards to the Revolution (as we call it) is a bit of an anachronism.
The Slave Trade Act of 1807 merely outlawed the buying and selling of slaves - it did not end slavery nor free any slaves on British soil. Furthermore, in 1785, Lord Mansfield (who presided over Somerset v. Stewart) ruled that "black slaves in Britain [were] not entitled to be paid for their labour." It wasn't until 1833's Slavery Abolition Act that slavery was eliminated entirely* from the British Empire.
Slavery in the United States probably would have ended at more or less the same time as it did in England had it not been for the Cotton Gin. By reducing the time it took to process cotton bolls from hours to minutes, the profits from the cotton industry soared; the labor effort needed for cotton production shifted entirely from processing to harvesting and as a result the number of slaves nearly doubled from 1790 to 1810 increasing nearly six-fold, some 3.95 Million or ~12% of the population, by 1860. This contributed immensely to the creation of the material wealth from which the US *still to this day* benefits. It's also what primarily fueled the resistance to abolition - there was simply too much money involved.
*Except for slaves owned by the East India Company plus a few other exceptions - though these were ended by 1843.