By Sonia Elks LONDON, Feb 5 (Thomson Reuters Foundation) - Rising temperatures pose a threat to the health of almost 12 million Britons, with the elderly and those with conditions such as diabetes and heart disease most at risk, a coalition of researchers and campaign groups said on Friday. Last year was Britain's third-hottest on record, causing some 2,500 heat-related deaths - a number that could double by the 2050s unless action is taken, said a report by The Climate Coalition and the Priestley International Centre for Climate. Failure to act swiftly on climate change "will spell disaster not only for our natural world, but for public health", said Clara Goldsmith, campaigns director at The Climate Coalition, which includes more than 140 organisations.