Britain's FTSE 100 index is to open 26 points higher at 7,618 on Thursday, according to financial bookmakers. * SKY: Comcast Corp on Wednesday raised its offer for Britain's Sky in a deal valuing the pay-TV group at $34 billion, challenging a raised bid from Twenty-First Century Fox. * BHP BILLITON: BHP Billiton Plc, handed in a proposal for a new labor contract to the union at its Escondida copper mine in Chile that includes a salary readjustment linked to