I'm guessing contracts signed that are bound by the laws of England and Wales.
Such agreements might or might not have clauses that stipulate in the even of disputes the parties would participate in legally binding international arbitration.
I'm guessing that in the event of cancellation of the agreement and non-payment of any stipulated penalty fee, that the Russian gas company would be able to pursue the debt through English courts, which could potentially result in bailiffs turning up at company premises to seize goods up to the value of the debt, alternatively the Russian company could apply for a winding up order and potentially put the company into administration if it didn't have sufficient funds to pay the debt.
So, basically, it's not possible to just cancel the contract and say fuck off, you're not getting your money. There are legal avenues that a Russian gas company could use to pursue the debt.
And just off the top of my head, I'm sure people with better knowledge of law/finance could think of more reasons why it's not that simple. (Unless the government passed a law that was retroactive to facilitate that kind of thing?)
Big companies, especially those that operate internationally, don't really want to start setting precedents of big multinational companies screwing one another over, lest the whole system collapse in a hot mess of contract cancellations, non-payment of penalty fees, fuck yous, law suits, international arbitrations, companies going to the wall, etc.