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Ok, you got the I-have-a-new-route-to-try bug in my head and couldn't shake it off, so went out and rode 75% of it, the remaining bit is the Icknield Way and the Roman Road, which needs no checking. Not only it's rideable, it's ace! I love it, it's up there in the top 10 of my routes from London. Only a few tweaks to make (already made in the version below). No gates, nice mix of bridleways, farm tracks and singletracks, not much tarmac, no major climbs.
I rode out and back in a loop, Strava ride here, with many more pictures. Both ways are feasible to Cambridge, but the return leg of the loop has more farm tracks, not the best on a gravel bike, I'd recommend the route below as is.
Full disclaimer, I simply drafted it after you asked, never rode it, review the route as much as you can and have a few backup plans (can always follow the NCR 1, thne 50, then 11 to Cambridge).
As you've learned RWGPS is your friend.
But still let me know ;)
PS. If it's post lockdown and you're looking for company, I'd be keen to ride it as well and come back on a train, when all of that will be normal again