durhamjen don't forget that slavery came about because these poor people were offered for sale by their own.
This process is still rife, the evident chains may not be there, but thousands are still being sold into slavery within that region, often by the same countries who were selling in 18c.
The UKs history of treating its own lower classes at this time was little or no better than those of the African slaves. Transportation to Australia, in the mines, in the mills and on the land, press ganged, however the misery and injustice of these practices is not as well publicised. The only saving grace in this instance is that they weren't sold by their own families, but they were often held in tithe.
If you study colonisation you will note that apart from slavery, there was a lot of movement of 'colonial' people during this period who moved of their own accord to work. There are still large colonies of Indians in the former African colonies, in the West Indies, Indonesia etc. who migrated during this period, these were the economic migrants of the day.