This is an introduced species into this country from France, hence its other name of French Partridge. There were earlier attempts but it was around 1790 that thousands of young birds were released and in some places now it is more common than our native Grey Partridge.
This spread has been helped by it's unusual habit of having two nests, one of which is incubated by the female and the other by the male. Given that each of these nests might have 10 - 20 or even more eggs in it, it is easy to see how it has spread.