In March 2017, Highways England updated its Route Strategies. The South Coast Central Route Strategy is the one that covers the A27 and A259.
Previously, in March 2015, the Highways Agency (as it was then called) published its first Route Strategy for South Coast Central:
which incorporated work from the Feasibility Study:
The Highways Agency split the strategic road network into 18 areas for the purposes of managing road investment, which can include maintenance as well as new road building. These areas do not always make sense on the ground with the route along the south coast split into several different areas. This approach was continued by Highways England.
To understand how they are approaching this work see the map.