I believe that the way pay is shown is a direct result of the recent pay re-structure (Not so relevant for Officers as they're all on the same regradless of branch). For ORs the supplement you are on has a core value depending on how well/badly your trade scored and x-factor is then added.
As for the proposed pay freeze by removing 1% of X-Factor, it wouldn't be as:
1% of £30,000 (core pay) = £300
14.5% of £30,000 = £4350 then 1% = £43.50
£300-£43.5= £256.5 pay rise instead of £300 (an actual .86% pay rise instead of 1%).