A Back in 2006 another Russian exile, Alexander Litvinenko, was killed in London. His tea was laced with Polonium-210. At the inquest into his death, a physics professor testified it could only have been produced by a single closely guarded plant in Russia.
Depending on your opinion, the British government either (a) did nothing because they were powerless or (b) held the matter in abeyance, until 2014 when Russian actions in Ukraine and Syria made drawing attention to Russian wrongdoing politically expedient.
The UK government has some other leverage - the UK is a popular destination for wealthy Russians. The government could stop turning a blind eye to people whose wealth came from foreign corruption, such as by applying new "unexplained wealth order" powers. For example, First Deputy Prime Minister of Russia, Igor Shuvalov owns two flats in London worth £11.44 million - while earning an official salary of £112,000
Of course, Russia has leverage too: 40% of Europe's gas supply comes from Russia. None of that makes it to Britain, but Britain's friends rely on it, and but plenty of Russian coal and diesel makes it to the UK. So there's no appetite in Europe to completely cut off trade with Russia.