Assuming you are talking about 4 October this is an airshow where IWM are not providing a free shuttlebus from Cambridge Station, and being a Saturday there is no ordinary bus service to the museum (normal service from Cambridge is on Sunday only).
The vestigial bus service #7a from Whittlesford Parkway mentioned by the IWM only runs on Monday to Friday.
My advice, if you didn't already have rail tickets, would be to change trains from Scotland at Peterborough, and take a train to Cambridge. Stay in Cambridge for the nights of 3 and 4 October and taxi to and from Duxford on 4 October.
I'm not sure whether or not that is still an option.
If you are able to change hotel and happen to be booked on an LNER train which calls at Peterborough that remains as a potential option.
Assuming it isn't and you are now travelling up from London you could take a Greater Anglia train from Liverpool Street at 28 and 58 minutes past each hour for Cambridge, get off at Whittlesford Parkway and pre-book a taxi.
The alternative is to take a Great Northern Cambridge semi-fast or stopping train from Kings Cross get off at Royston, and take a taxi. At least on the way back that taxi would need to be pre-booked.
Those trains are the 27 minutes past each hour stopper (overtaken by the 36 past semi fast), and the 36 and 6 minutes past the hour semi fast).
Greater Anglia is the cheaper route, and for various reasons would always be my preferred route. However there is nothing at all wrong with Great Northern.
It may depend on where your London hotel is.
You can walk from Whittlesford Parkway, but it is a somewhat sub-optimal walk, so is not the best of ideas.