American Air Museum

Train from Liverpool Street Station to Whittlesford Parkway
then take the 7A bus from Whittlesford Parkway to Duxford. (Monday to Saturday only).

leave enough time because there is a lot more to do at IWM Duxford than the American Air Museum - it is a fabulous place.

As you are south of Cambridge, you might also take a bus to the Cambridge American Cemetery and Memorial just west of Cambridge https://www.abmc.gov/Cambridge a peaceful, calm, beautiful, reflective place - a very moving experience.

The 7a only runs 3 times a day Monday to FRIDAY so you need to get your timings right- https://bustimes.org/services/7a-duxford-whittlesford-sawston-whittlesford-duxfo
(The National Bus Timetable Dataset shows that the Saturday service has been withdrawn)

The better service, surprisingly, is on a Sunday- the 132 from Cambridge Railway Station- https://bustimes.org/services/132-cambridge-duxford-saffron-walden

Or taxi from Royston or Cambridge Stations.

Unless it was an air show day I doubt you would find a tour. It's the kind of place almost everyone drives to.

I can't help with Duxford, but if you decide you need to substitute an easy to reach place in London for IWM Duxford, I can recommend The Battle of Britain Bunker in Uxbridge, at the end of a couple tube lines. RAF Fighter Command’s No.11 Group Operations Room during the Second World War. Taxi or walkable from Uxbridge station.

I made a tour reservation - that way you can be assured of seeing the underground Bunker. My understanding is that they sometimes get school tour groups, otherwise, not a widely known site to visit. The museum has lots of "personal memory" sorts of exhibits and explanations of how the communication system worked to keep the RAF apprised of incoming German aircraft and protect the South Coast.

I visited with tour and extensive museum time in about half a day from central London.

http://battleofbritainbunker.co.uk

We found Duxford amazing; the American Air Museum part was interesting, but there was so much more to it that interested me more. My husband enjoyed watching them work on some of the old planes, such as Spitfires. Hope you can get there easily. We stayed in Cambridge and took a bus recommended by Rick Steves at the time. It resulted in having to walk along a very busy road that had no shoulder or sidewalk. Hopefully that recommendation is no longer in his books.

The other option would be to take the train from Kings Cross to Cambridge (it takes less than an hour) and get a taxi to the museum. There is a taxi queue at the station. There is also the bus as mentioned above if the timing works for you.