By Amal S and Shashank Nayar (Reuters) – London’s FTSE 100 fell on Thursday with heavyweight financial and oil shares leading declines, along with miner Rio Tinto after it severed all ties with Russian businesses and also traded ex-dividend. The blue-chip FTSE 100 closed down 1.3% at 7,099.09 points, with oil heavyweights Shell and BP, as well as Rio Tinto and finanical firms HSBC and Prudential the top drags on the index. Shell dropped 2.4% after saying it faces further writedowns from exiting Russia, flagging that it had $0.4 billion in Russian downstream assets as well as the $3 billion in …