In addition to the taxi suggestion, depending on how long you’re planning to spend in Cambridge, you may be better off actually hiring a car. If you book in advance and spend some time searching, and don’t need a large car, you may be able to get a car for under £40 a day. Even adding the cost of petrol, if you’re going for a day, i.e. returning the within 24 hours of pickup, that may be fairly cheap and fast, too.
EDIT: To expand on Willeke’s comment, liftshare.com is a good resource too. Have a look: https://liftshare.com/uk/search/from/milton-keynes – put “Cambridge, UK” as the destination and see if you can find a reasonable match. If you’re going the other way, then start with https://liftshare.com/uk/search/from/cambridge and put Milton Keynes as the destination.
EDIT 2: To address the second of Willeke’s comments, there are also car share schemes that you may be able to utilise. Zip car is the most popular one, but there are also Virtuo, EasyCar club, E-Car club, Hiyacar and a number of others. I haven’t check which ones are available in Milton Keynes.