The bad way to do it is to simply put a blob indicating an allocation.
This prejudges the form of the city.
The form should follow a masterplan.
A mistake made with some of the new tows was confining the masterplan to a determined in advance development corporation area, which the ministry later admitted was a mistake and rectified in the third gneration new towns.
Given that they are unlikely to commence within 5 years the best way is not to allocate land but simply show the location indicatively on a key diagram with indicative completions levels for later years of the plan and not for first 5 years (hence not needed on policies map). The policy should state the exact location will be subject to a study and masterplan to be adopted as a DPD.
Then commission the masterplan and include as an allocation.
Of course the preliminary feasibility work can and should be part of the evidence base on the local plan in the first instance, so the two should proceed in parallel.