It's plausible that the P14 can be a design exercise (never to go onto being a commercially available public offering) or the P13's roadster/spider counterpart.
Just thinking out-loud, as'per McLaren Automotive's past projects & their leadership's initiative to bring forth a new-model and/or derivative every year.
The 12C was a '12 model-year offering, then '13 model-year was the 12C-Spider, '14 model-year being the P1, '15 model-year being the P13 "baby 'Mac", to be followed by '16 model-year being the P13 roadster/spider, and then '17 model-year for the 12C successor. That'll allow for a standard 5-year model cycle.
In clearer terms...
'12 - 12C released
'13 - 12C-Spider released
'14 - P1 released
'15 - P13 "baby 'Mac" released
'16 - P13 "baby 'Mac" roadster/spider released
'17 - P14 12C successor released
With all that said, I wouldn't even be surprised by a mid-model 12C refresh/change that can be a facelift of sorts along w/ some other attributes beyond the v.1X upgrades. Obviously, I have nothing to base any of this on, just thinking how the existing platform can remain "fresh" & in the limelight...
(would make most sense to me, having in the range a coupe - spider - streetable track/GT3 derivative)