TBH (from experience at quite a few UK airshows!)
- At that distance, even a 300mm lens is going to produce a
very small image of the aircraft itself (even something like a 500mm lens would only get you a half decent shot).
- I usually use around 180-250th shutter speed to get decent prop blur on Spits and similar - but maybe 125th or so for the older & slower biplanes - but that'll be hardly noticeable at that distance anyway!
Frankly, you need to get a
lot closer to the predicted flight path to get decent results with the kit you are intending to use - at airshows, the guys who get the really decent shots are those standing right at the front of the crowd line, and using Canikons and
massive (600mm+) lenses!