Running

Manchester is what I was told, and having run it can confirm it is flat indeed. Also not a coastal one so less subject to prevailing winds

I don't know about the absolute flattest but Edinburgh is downhill overall with only one or two small uphill sections and most of it is along the coast which is obviously pretty damn flat. If you're trying to qualify for something I'm not sure whether or not it counts though (something to do with it being downhill overall). Also as noted it is more exposed to headwinds being on the coast.

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

Abingdon is pretty flat