I don't know whether there really are more MX5s in Edinburgh, but that was my impression from being there for a couple of days last October. You'd see two or three an hour, rather than one every few hours where I live (Essex).
As others have pointed out, Great Britain is pretty much the optimum latitude for open-topped cars. If the sun is blasting down all the time, it gets uncomfortably hot an what you need is air-con; if it is below 10 deg C most of the time, it is too cold. What you really need is daylight and a moderate temperature. And we Brits have to make the most of what little sunshine we get.
The only explanation I can think of for the prevalence of MX5s in Edinburgh v Essex is the much longer evenings in summer. Assuming everybody gets home from work around six o'clock and likes to go for blast in the evening, for six months of the year, you get no daylight whether you are in Edinburgh or in Essex. Whether it got dark at three o'clock or at four o'clock is neither here nor there.
But during the summer, in Edinburgh you get up to an hour more daylight than in Essex; that makes a huge difference to the number of useable hours.