The term 'affordable' here has its own definition - one of many - to mean average house prices are less than 4 times the average wage.
In simple terms if the average wage is £26k then if the average house price is more than £104k then it is not affordable.
Interestingly the key workers mentioned above - police, teachers, nurses, fire fighters etc on average receive well above average wages and im not includign London weightings. Typically the average key worker salaries would be 20 - 50% higher than the average wage in the £30 to 38k range
As such if the affordable definition here is 4 times salary compred to house prices then even Clacton and Portsmouth are too expensive to afford to live for those on average earnings.
The survey is thus highly skewed and ALL of the south east (and most of the UK) is unaffordable for those on average earnings at £26k or so per annum.