Those tactics have been used in Russia, Venezuela and Argentina. But those aren't the greatest rule of law states. In Britain all of the tactics you describe would be challenged in the courts and struck down given that they are nakedly predatory renegotiations in bad faith. In order to make any of them work at all, the first step would therefore have to be a major rewriting of UK contract law, granting powers to renegotiate in such a way to the state on some sort of strategic basis, or under some other doctrine of unusual force outside of emergency.
Unfortunately, given that similar contracts are used to raise all the sovereign borrowings that make our government work day to day, the instant effect of even presenting such a bill to the House would be to bring the UK's sovereign credit rating down to the sort of status enjoyed by countries who break contracts and expropriate assets outside of emergencies. Which would be ironic, because then we get an actual emergency.