The London system is easy, and it's actually designed to encourage people to vote for who they want, not who they must.

Your first vote goes to the person you really want to see in power. Your second one goes to the candidate with a realistic chance of winning whom you would dislike the least.

So vote Paddick (or someone else) in the hope of getting someone other than Boris (lorks, what a hoot!) Johnson or Ken (I'm so progressive I'm best mates with a religious extremist) Livingstone. And then give your second preference to Boris or Ken if you must.