Generally, unless you are a military venue or a major event, your chances of a flypast are likely to be based on whether there is a plane in the air nearby visiting such a place, so the more flexible you are in requesting a date for your smaller event, the greater the chance of the plane making a slight detour to visit you.
Woolwich Barracks is about a 15 to 20 minute walk from Woolwich station(s) and is pleasingly next to a large open field, so there's going to be plenty of space to see them fly overhead. Be aware that the field right next to the barracks is their sports ground, and the footpath through it can be closed at times - but the large Woolwich Common next to Ha-Ha-Road is open to the public.