'Exactly. The labor-surplus of North American/South American/South Asian/African slaves, plantation serfs, indentured laborers, coolies etc. was extracted by British capitalism for centuries, with near-genocidal consequences for the extractees. Liberal Brit capitalism = illiberal Brit Empire.'
It is true that British capitalism used slave money to get going, that it used the anti-slavery campaign as a cover for colonial conquest http://www.monthlyreview.org/mrzine/heartfield260107.html and that indentured labour continued to be a means to seize raw materials by force into the 20th century. On the other hand, free labour was a lot more productive for capitalism than forced, which is why Marx relegates the discussion of slavery to Capitalism's pre-history, seeing free exploitation as more typical.
That is why Europe's Trentes Glorieuses coincide with a - very destructive - repatriation of European capital from the colonies back home.
Also, you should know that the fifty year gap between abolition of slavery in Britain and in America has meant that forever more historians over here teaches that slavery was an American vice.