As of a few months ago, the UK was still discussing separating from the ECHR [1], so they may yet snatch their success from the jaws of failure either way.

Reading between the lines of their Q&A [2], it seems largely de facto and functionally voluntary for the participating nations to comply, perhaps straying into strongly-worded letter of disappointment territory in the case of a failure to comply. And they can apparently just leave. How do you push around a sovereign entity with a military? Sanctions, I guess.

[1] https://www.ibanet.org/UK-ministers-continue-to-discuss-dras...

[2] https://www.echr.coe.int/documents/d/echr/50questions_eng