It’s not about guilt. It’s about whether or not you know the password. In most cases not knowing the password will be deemed improbable because the chances are extremely low. You’ll have to provide a scenario where it becomes probable, and said scenario must be deemed probable as well.
https://en.wikipedia.org/wiki/Regulation_of_Investigatory_Powers_Act_2000
Well reCaptcha v1 was used for the digitization of books. And that they proudly talked about.
But to be honest, the pictures were in fact used to dether bots. But also to teach selfdriving cars. I think I also remember a time they used to ask to fill in house numbers probably for their Maps accuracy.