here's the thing, some like Rishi Sunak and Kier Starmer who ar very powerful people still support this, if such a blackout got enough attention they would HAVE to comment to their constituents as to why half of their favorite UK-based websites cannot be accessed at this moment and if they can undo this/ explain to them why this has happened, otherwise those exact same constituents might not want to vote for them come the next general election. the Senators who sponsored SOPA withdrew their backing because they were clearly disturbed by what their constituents were saying and they wanted to secure their re-election prospects so they did something they thought could get a few more votes.
letter-writing issint enough, at best there might be some rumbles of change and at worst they'll be thrown out as spam. we need to take direct action at these MP's NOW in a non violent way, make them truly feel vulnerable and second guess their opinions on this matter and this is a time and tested way to do that.