If you have mated queens you can make up nucs even in to October, you need to give them feed and extra stores, but its entirely possible. If your relying on cells mating, then mid august is really your latest, but thats us her in northern France, very similar to south coast of the uk. Drones get mostly binned here first and second week of September.
Its all about having good colonies to split, being right with your timings and covering yourself if the weather goes bad early. A lot is possible but you need to be well organised and realistic in what you split.
Ive made some goofs, but you learn! this year i am actually starting my splits in about 3 weeks and giving cells. Saving our mated queens (from mating nucs) for requeening hives in my main apiaries where necessary.
Any nucs not queen right will then get the last round of mated queens from the mating nucs as we close those down and bulk up on stores for the winter. Any further down the line will just be combined.
Is it really nearly that time already lol where is this year going!!