Comcast has gatecrashed Rupert Murdoch’s takeover of Sky with a rival £22bn offer, sparking a bidding war for Britain’s biggest pay-TV broadcaster. The media and telecoms company, which owns NBC Universal and is the largest cable operator in the US, has made an all-cash offer of £12.50 a share, a 16% premium on the offer from Murdoch’s 21st Century Fox, which values Sky at about £19bn.