Boston (uk) claims to be pretty damn flat, but you’ll be a bit lonely!
There are loads of niche marathons with zero altitude gain, there was one at dorney a few weeks back that went up and down the 2.5km flat side of the lake
Its so flat, so dull and mega windy when it wants. It was where I trained last time I did anything seriously and its a great place for running if you like treadmills.
The flattest mara in UK - where is it?