Sillybilly
It is the Environment Agency that produces the flood maps. However, planning rules are very strict about flooding and planners use the EA maps - it is nothing to do with the local council designating which areas are flood plains or not. I'm guessing that the areas people think are flood plains aren't actually at all but I could be wrong.
That's the kind of sophistry I meant. EA provides a map. Flood risk assessment fall on the developer www.gov.uk/guidance/flood-risk-assessment-for-planning-applications and a council/planners will not put the permanent tag 'flood plain' on any areas 1, 2 or 3 in order to prevent development. They can always choose to allow a developer to 'manage' the risk.
So in Tewkesbury, for example, there are 3 very large developments on land that always floods, year in, year out, are/were Zone 1, flood every year to a lesser or greater extent. They all have flood defences, have been built on natural or built ground that is scant inches above the 'agreed' high water mark. They even have flood management built in a couple, to prevent the water that would have flooded that ground going elsewhere... but it just happens further downstream... as shown on the national news last night!
Your faith in planners is touching, sadly, as many people round here are only too aware, it is misplaced!