People not tech
It's the yanks that go for exotic technical solutions and as others have said, attached devices to undersea telephone cables to bug Russian military communications. The FSB (formerly KGB) has tended to use people. They never seem to have trouble finding people in positions of responsibility who will do their bidding either for cash or political idealology. If the SIS are really paying under 20K to some of their systems and networking people working in central London, I wouldn't be surprised if some of them were moonllghting for the FSB.