Regarding PC-21, that aircraft is used by some air arms as an ab initio trainer and by some as an advanced trainer leading onto Gripen, F-18 etc. Therefore, it covers the whole spectrum of training. When the contractors (Affinity and Ascent) decided on platforms for MFTS the Hawk T2 was already well in service and the PC-21 would have had considerable overlap of capabilities with it. In fact, the PC-21 has some capabilities which I believe the Hawk T2 does not (synthetic air-to-ground radar). The PC-21 was more expensive than the T6 and so was not a sensible option, however good and capable it may have been.
I was involved with the proposal by Affinity to Ascent of the T6 as the platform for this role but my remit did not involve any aspects of AEA. Both the T6 and PC-21 are fitted with Martin-Baker Mk 16 ejection seats. Therefore, it begs the question why the UK T6s were not fitted with a version of the seat that used UK AEA. I suspect that it was a question of cost but some here may know more.