Was I lucky or is Sept as good as it gets
It depends exactly when in September. September is on average a wet month, but there is often a dry spell in early September. June is on average a dry month, but there is often a wet patch. March-May is the driest three-month slot of the year, but Mar-Apr is a bit of a risk because its still cold with risk of snow in the hills. And of course it can rain any time.
What actually happens in any given year is very sensitive to where the jet-stream ends up that year. Summers tend to be either wet or dry depending upon where the jet stream sets itself up for the period. And this can vary by how far north you are in the country - the south can have a wet summer and the north a dry one, and vice versa.
Because the jet stream does move to the south in the winter, there is a tendency to have a wet patch in late spring or early summer as it heads north (often in June - sometimes coming a bit late and disrupting Wimbledon), and then to have a dry patch in late summer or early autumn (often in early Sept) as it heads south and brings in the rains of autumn. But the thoroughly rained out June we had last year was because the jet stream was in quite an unaccustomed config altogether.