Except multiple European countries have derogated from the ECHR and no-one has batted an eyelid
They can insist their judgements are carried out.Indeed, they have no real power other than bad PR. The judgment is worth a read. I'm amazed our resident self appointed Human rights experts have read it already.
They can insist the 'out-lawed' actiosn are reversed and damages paid. So the asylum seekers sent to Rwanda would have to be returned to UK.
If a signatory persistently refuses, they can be chucked out of the ECHR.
I ssupect it would depend on the seriousness of the issue. I would imagine a gross disregard for Human Rights would figure pretty high on that scale.