a) It seems important that this is a national event, not just a London one, and supporting that is worthwhile.
b) I'm lazy.
I find it hard to put into words how awful it is that even from my relatively privileged position in society (i.e. well educated, prosperous, white, straight) I'm more scared of my own government and its security services than I am of the forces they claim to be protecting me from. I'm far from a tin foil hat wearing libertarian wingnut, I believe that structures of state are necessary for a fully free and equitable society, I want effective and strong government. However, I also want to be able to live as a private individual, have private communications, freedom of speech, freedom of protest. I also want the legal ability to effectively protect myself from any potential injustice an individual, a corporation, or the state may perpetrate.
It's bizarre that the House of Lords, a largely unelected body which reeks of patronage, and has aristocratic and theocratic remnants, has time and again proved the best bastion against the situation being even worse than it currently is. A Lords committee recently published a report on privacy, whose recommendations opened with:
We regard privacy and the application of executive and legislative restraint to the use of surveillance and data collection powers as necessary conditions for the exercise of individual freedom and liberty. Privacy and executive and legislative restraint should be taken into account at all times by the executive, government agencies, and public bodies.. Unfortunately, I've no faith at all that this report will have any useful effect.