I think I just wore my normal daytime cycling kit for the last one, ie short sleeved jersey, shorts, and mitts. The one prior to that, I had on a long sleeved jersey, and was glad when we hit some patches of cold mistiness.
For September's, if the forecast isn't much different from now, I'll wear the short sleeved jersey again, and just pop a long sleeved thin fleece top in my bag, just-in-case.
On the other hand, I seem to run a bit hot than many people, and regularly cycle in the late evenings, so know what to expect. As others have said, look at the temperature before you leave, then knock a couple of degrees off for being somewhat later, and then a couple more for being outside of the Urban Heat Island of London.