Biblical Covenant Breaking and English Law

Having made a covenant with God in Exodus 24, the people abandon their covenant obligations in Exodus 32 by making and using an idol, rejecting Moses and indulging in gross sexual immorality, perhaps amongst other things. They have not merely sinned. They have broken the covenant such that they deserve death, as God says.  They are Covenant Breakers not just sinners.

UK contract law can provide an illustration.

If I understand it correctly, which I very likely don't, in older law, their actions might have been seen as a "fundamental breach".

Today it might be specifically called a "repudiatory breach". That is, not that one party explicitly announces their rejection of the contract (necessarily) but that the actions of one of the parties are such a serious breach of contract that the wronged party is entitled to terminate the contract (and to receive damages). That is what God considers doing, but Moses persuades him to have mercy and stick with this people, although they face serious punishment.