By Andrew MacAskill and Michael Holden LONDON (Reuters) -The British government said on Thursday it had frozen assets worth more than 18 billion pounds ($20.5 billion) held by Russian oligarchs, other individuals and businesses sanctioned over Moscow’s invasion of Ukraine. Russia has overtaken Libya and Iran to become Britain’s most-sanctioned nation, the Office of Financial Sanctions Implementation (OFSI), part of the finance ministry, said in its annual report. The frozen Russian assets were 6 billion pounds more than the amount reported across all other British sanctions regimes. Russian bi…