Given how the govt goes about consultation, they probably only asked large British sites, who would notice if traffic dropped from a major ISP.
The problem is greatest lies with information sites, especially non-profit and foreign ones, who may not notice that a single British ISP has blocked their site.
Of course this is an important category for many visitors, but I suspect the government does not regard them as important.
How is the blocking done? DNS? IP? Transparent proxy? A combination?