doesn't have to be, mind you it is rather boring, but there is NCN 5 along the coast which is flat other than 1 short rise which can be diverted around.
Getting back again via another route is another matter. My OH did a 158 miler (was meant to be 200 miles but he baled) back in the summer from Northwich to Snowdon and back again and the only real climb he had was up Pen y Pass
My first 100 miler only had about 3,000 foot of climbing in it.
https://www.strava.com/activities/164502812 that was north wales and Cheshire area. It wasn't designed to be flat and I had no idea what the route was because a friend planned it and road with me, but it is relatively flat, though some of the roads were busier than I would have normally ridden on.