I don’t remember seeing this campaign. Are there any links?

This isn't the same ad as described in the article, but is a government funded Saatchi one from around the same time: https://www.noplacetohide.org.uk/ > The campaign is funded by the UK Government and has been developed by a steering group of child safety organisations with support from M&C Saatchi.

I think that was the website displayed on the adverts.

Some anonymous entity has been putting up posters in my neighborhood decrying Apple for "allowing" CSAM to be stored on iCloud. Very creepy propaganda campaign.

Those posters are almost certainly plastered up by someone who genuinely believes in the cause. They have been manipulated by corrupt media, but it is a tactical error to assume that a political party is directly paying for such poster campaigns.

It's also guerilla marketing 101, and they wouldn't be the first company, interest group or marketing agency that used those tactics to promote their products, interests, etc.