I contacted my MP about this last week, and he emailed back, pledging his opposition to this plan. I'm unsure what I should be doing next. Is there anything else I can do?
You can contact your MEP (and other representatives) at (http://www.writetothem.com/). This sort of thing often pops up in Europe and it would be a Pyrrhic victory to stop it in UK and then it gets made mandatory across Europe.
You can try and convince those around you that this is an issue and that they should do the same. Parents, friends, but perhaps also work colleagues and clients. (Any tips here would be welcome, I am a developer, not a salesman, so I just get "here he goes again" rolled eyes)
You can write/contribute/donate to organisations like Liberty or EFF that fight this sort of encroachment on our lives.
You could also get more active in politics yourself. I hear the UK Pirate Party is always looking for support.
Talk to as many people about this. It's pretty easy. One way is this: "have you heard about <topic>?" Then explain away.
Look for as many campaigns on this issue. There's likely several. Find the most simplest one, this will be your backup card if some people don't understand why you're upset.
Contact as many media outlets possible. Explain your issue with it.
Social media push; Tweet relevant campaigns, post and explain to people on social media (reddit/digg/forums/etc) on what to do.
Join a political party. They should typically be aware of a lot more issues and may know of ways for you to help. Sometimes the party needs someone to do a campaign, graphics, etc.
Remember: you don't write to change anything, you write to establish that this is not ok and gives ammunition for the people arguing against this.