(Reuters) -UK’s main stock indexes plunged on Thursday, tracking a sharp selloff in global equities, as investors shed riskier assets after Russia launched an all-out invasion of Ukraine by land, air and sea. The benchmark FTSE 100 index sank 3.8% by the close, while the domestically focussed mid-cap index fell 2.6%. Russian forces invaded Ukraine by land, sea and air in the biggest attack by one state against another in Europe since World War Two. The United States and its allies will impose “severe sanctions” on Russia, U.S. President Joe Biden said. Russia-exposed miners such as Polymetal, …