It wasn’t particularly surprising that Tim Taylor would call Michael Payton and ask if he wanted to meet for coffee. Two of London’s most renowned insurance lawyers—and two of the most senior names at leading U.K. insurance firms Barlow Lyde & Gilbert and Clyde & Co, respectively—the pair have been working together on cases for more than two decades. So when Taylor contacted his old friend Payton in April, Payton readily agreed to catch up at the Costa Coffee below AIG’s European headquarters on Fenchurch Street. But then Taylor asked if he could bring along BLG senior partner Simon Konsta. “I thought ‘Gosh, what’s going on here?’ ” recalls Payton, who has led Clyde for 27 years.

Less than four months later, the partnerships of both firms voted to approve an audacious merger. When the combination formally takes effect in November, it will be the largest-ever merger between two U.K. firms—bigger, even, than the 1987 formation of Clifford Chance or the 2000 union to create Denton Wilde Sapte (now SNR Denton). Combined revenues of almost £310 million ($479 million) will see Clyde overtake the likes of Berwin Leighton Paisner, Simmons & Simmons, and Ashurst, placing it on the cusp of the U.K. top ten and creating the world’s biggest specialist insurance firm.