In many cases they hardly "decided"; they were en masse kicked off the land, so had no real choice. See: http://en.wikipedia.org/wiki/Inclosure_Acts
Mostly I don't think it's at all clear that "being poor in the city working in a factory" would actually be preferred over "being a peasant farmer" given conditions of the time, if it were truly a free choice. In the United States at around the same time, you find many people choosing to leave cities to become subsistence farmers on the frontiers, because the Homestead Acts made it a viable option. If the UK had taken the enclosed land and distributed it via a Homestead-Act style process, would British peasants have chosen that option?
This being said, I think we agree on two points: the Enclosure Acts increased the pace of urbanization (but I believe it was by providing cheap food, not forced relocation), and they also increased the prosperity of society in aggregate. On my side, I argue that this prosperity registered a net gain for both landowners and the poor classes.
And people in the east end of London were most certainly not well fed: http://london.sonoma.edu/Writings/PeopleOfTheAbyss/chapter25...
Please try not to be so knee-jerk dismissive of everything that doesn't fit into your free market viewpoint.