Whittlesford Station is 20 - 30 minutes walk from Duxford, but I've no idea how many trains stop there. It's far from being a teeming metropolis.
I believe there used to be dedicated buses from Cambridge railway station on airshow days, which I imagine will have many more trains, but I only know that from seeing signs as I drove past.
Duxford is the thick end of 50 miles from London, which I know is just around the corner by North American standards, but is a long way here. The cost of a taxi, of any sort, I suspect would be astronomical.