Just to further answer some of our enquirers below. When we call for a write-in vote for socialism (what could be called a spoilt ballot paper) we are not doing so because "electing Johnson will hasten [capitalism's] demise?" That would be a perverse and monstrous policy.
We want a write in vote, or a vote for a socialist candidate precisely because that is the speediest and most direct way of abolishing capitalism. We understand that, however misguided or foolish politicians are, there are limits to what they can do - mostly they can only alter the mood music of what the market demands. Voting for Ken, Boris or Brian won't get you them as mayor, only King Capital with a different sock puppet for a mouthpeice.
In the past, radical movements, such as in Argentina, have managed to use a spoilt ballot - effectively a political strike - to overthrow entrenched corruption and political malaise. We do not consider it to be a passive or ineffectual option (as Londonsocialist has just written, write-in votes will appear on a big screen, and be viewed by candidates and agents, imagine them wading through a million spoilt votes...)
I'm not lucky enough to live in Lambeth & Southwark any more, so I won't be able to cast my vote for a socialist candidate, but I most certainly will go to the polling station and cast my vote for world socialism, and nought but.