You get two votes - and if your first preference is knocked out, it gets allocated to your second preference.
You might want to use your first preference to vote for some of these guys:
Rejoin the EU party, to let them know you care,
the Reclaim Party because "it's political correctness gone mad" is your personal catchphrase,
UKIP to stick it to the Remoaners and wipe their "I told you so" smirk off their face,
the Heritage Party if you think climate change is a hoax and really hate bicycles (could be popular here!),
Let London Live because we haven't given the Corbyn clan enough of a chance to bugger things up,
Real London, the Brian Rose self promotion vehicle, because, you're Brian Rose (can't think of any other reason!)
Sian Berry of the Greens, because she's decent, wants London to be the new Brighton and is almost as pro-bike as Boris,
Count Von Binface, because his manifesto* actually makes sense and you're someone whose answered Jedi for the last 3 censuses.
When they get knocked out, your second preference can go to Sadiq or Shaun, although given that Shaun's own party has given up on him, I don't know why anyone else should bother.
*Highlights of the Binface manifesto for me:
Hammersmith Bridge to be repaired, and renamed Wayne.
Speaker phones on public transport to be banned.
London to join the EU.
The Trocadero to be turned back into a truly top-notch video arcade.
Ed Saper ● 2033d