Hopefully I can make this.
Here's a few suggestions (one is in Kent, but maybe for the future?). Both are pretty flat on smooth quiet roads, all left turns.
Essex: super quiet, no roundabouts / lights / residential areas, but a touch narrow for a short section
Kent: a few villages, but easy to get round fast, and one set of traffic lights. really wide open roads with good visibility
There must be loads more in Essex, will keep thinking