You need to have accepted a PA pay spine offer BEFORE you can apply for the FRI.
Here's an interesting question though: what if someone accepts PA, thereby loosing their 38pt option, but then PVRs aged 37yrs and 6 months? Will they still transfer onto the PA spine on their birthday as planned and do their last 6months on full pay? Or will the PA offer be declared void, in which case does their original exit date on their 38th birthday get reinstated?
Just stirring!