By Sruthi Shankar and Amal S (Reuters) -The UK’s top share index rose on Thursday on the back of gains in banks, but shares of utilities slumped after Britain announced a 25% windfall tax on oil and gas producers’ profits. The blue-chip FTSE 100 was up 0.6%, with banks leading the gains with a 1.4% rise, while the domestically focussed FTSE 250 index inched 1.6% higher. Facing intense political pressure to provide more support for billpayers coping with what political opponents and campaigners have called a cost-of-living crisis, British finance minister Rishi Sunak said energy firms were maki…