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When I've had a full day I've used cycle.travel to plot a route back from Harwich (roughly NCR 51 to Colchester then NCR1 and lanes to Chelmsford, then lanes to Upminster and follow the cycle routes into town); it's mostly laney and pleasant, but next time I think I'd rather get the train back (either direct from Harwich, or ride to Manningtree and get the fast train back from there) and have an extra day's touring in NL.
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https://ridewithgps.com/events/157661-tandem-to-amsterdam#routes/36915366/preview
Full route is here. Is fine back from Harwich, in the main, however would make some changes arround Colchester.
There's a couple of off road bits in there however they were fine. It's the 40mph stuff that I'd change. Safe enough though. I rode the trip with my brother back in 2016 with the same route.
We caught the overnight ferry to Harwich then rode to Chelmsford to break the journey up for the children. Back to London today.