Dear Polarised,
When very early cars were built with negative earths (ie. the negative terminal of the battery connected to the chassis) it was found to cause excessive corrosion of the battery terminals compared to positive earth cars. Positive earth cars also required 10% less voltage from the ignition to produce the same spark. British car manufacturers changed to the then superior positive earths but European and American manufacturers retained negative earths.
Improvements to battery construction materials reduced corrosion to manageable levels and modifications to coil construction enabled the high tension circuit to be positively earthed even with a negative earth system. The need to compete on the world market forced many British car manufacturers to later revert to negative earthed systems.
