It has a "Spit14" icon, that means it will be a perk plane. That is as it should be. Unperked, the Spit XIV would be far, far too common.
The British planeset is already stacked with low altitude fighters (Hurri I, Hurri IIc, Hurri IId, Mossie VI, Spit Ia, Spit Vb, Seafire IIc, Tiffie Ib and Tempest V), the Mk XII would perform no better than the Mk XIV down there, and it would be terrible at altitude. The Mk XIV adds a high performance at altitude fighter to the British planeset.
The Spitfire Mk XIV was never eqipped with 4 20mm cannon.
The screenshot was posted by Grayarea from Apache's film of the Spit14.