Sam Holloway said the following:
You can certainly buy tickets from Ely that start at Cambridge (e.g. if
you hold Ely-Cam season ticket and want to travel to London, you can
buy the Cam-London part from Ely).

.... and if you do that it's always worth bearing in mind that your 'gold
card' season ticket *counts* as a network rail card, so gives you a
third off for off-peak travel. And you can get that discount for up to
three (I think) people travelling with you, too. And you can each
upgrade to first class for £3 (subject to various T&Cs). The folk at Ely
train station tell me that I'm about the only person to ask for the
discount ... which might have something to do with it not being very
well advertised...

Stuart.