First, it has to be noted that the traditional mystery originated in America, though it failed to make an immediate impact there (or anywhere else for that matter) The genre then grew and matured in Europe before finally making it home in the late nineteenth century, thanks in large part to the runaway success of a certain Sherlock Holmes. If American mystery writers at the turn of the century were greatly indebted to Conan Doyle, they soon found their own voices and a distinctively American school emerged.
Second and consequence of First, the Golden Age was not a solely British affair. The Americans too craved for great detectives, convoluted plots and quirky modus operandi - and local authors were all too happy to oblige. As difficult to believe as it may seem to us now, Philo Vance and Thatcher Colt actually coexisted with the harder-edged Sam Spade and Race Williams. American Golden Age fiction tended to be more outlandish and less serious and "protocolary" than its British counterpart; some of the most innovative, original and sometimes radical Golden Age mysteries were written by Americans.
Third, British crime fiction between the wars was much less uniform than the perusal of the Crime Queens would have one believe. The Detective Novel was not the only form competing for readers' attention; it had to deal with a cruder yet extremely popular rival, the Thriller. Now most often associated with the blood-and-thunder stakhanovist Edgar Wallace, this genre was actually extremely varied in style and tone - and some thrillers (David Hume's Mick Cardby novels for instance) with their emphasis on action, violence and somewhat confused plotlines bore more than a faraway resemblance to hardboiled fiction.
Only after WWII did the lines become firmly (or more firmly) entrenched. Both the American and British school sensed a need for change, but they reacted differently. While the Americans ushered in the newfound territories of hardboiled and psychological suspense, the Brits mostly stuck to their guns - fallible police detectives replaced amateur detectives and the setting moved from the upper class to the lower ones, but the traditional apparatus remained - only in the Nineties would hardboiled/noir become a significant force on the British mystery scene.