By Shashank Nayar and Bansari Mayur Kamdar (Reuters) – London’s FTSE 100 slumped on Thursday, as volatile crude prices sent heavyweight oil stocks sharply lower, while investors feared the impact of the fast-unfolding Ukraine crisis. The FTSE 100 index settled 2.6% lower, as oil majors Shell and BP fell 5.7% and 4.3%, respectively, tracking a drop in crude prices on a potential Iran nuclear deal that could boost supplies. [O/R] Russia-exposed miner Polymetal plunged 42.1%, after losing its spot on the index. Further losses in the benchmark index were capped by gains in exchange operator London…