Britain’s top share index rose on Wednesday, boosted by its heavily weighted mining sector after Federal Reserve Chair Janet Yellen urged caution in raising rates, lifting riskier assets. Miners .FTNMX1770 were up 5.5 percent, rising off a 1 month low hit in the previous session. The sector had fallen 15.8 percent since hitting 4-1/2 month highs earlier in March, pressured …