By Amal S and Shashank Nayar (Reuters) – Britain’s blue-chip and mid-cap stock indexes recorded their worst week since March 2020 on Friday as concerns over the impact of Russia’s invasion of Ukraine deepened. The blue-chip FTSE 100 index ended the session 3.5% lower and was down 6.4% over the week, with precious metal miners, automakers and travel stocks were the main losing sub-indexes. Russian forces in Ukraine seized Europe’s biggest nuclear power plant on Friday in what the U.S. said was a reckless assault, although a blaze in a training building was put out and officials said the facilit…