The differences between the two three versions are readily accessible everywhere - in essence the T.2 (the subject of this project) being the glass cockpit fast jet evolution to prepare RAF pilots for the glass cockpit Tornado and F-35 aircraft they'll be flying operationally. It's progenitor - the T.1 and T.1a entered service in the very late 70s/early 80s as the RAF's fast jet trainer to replace the Folland Gnat and eventually the Hawker Hunter. This version is the subject of the JF Hawk T1 payware model. McDonnel Douglas got interested in it in the 80s and developed their own version to replace the A-4 Skyhawk and T-2 Buckeye that became the T-45, fully carrier capable trainer. That version is the subject of IFE's T-45 payware model.
I think the narrative that one needs to buy two payware models to get a freeware mod misses the point. People who really like the Hawk platform in the sim likely have one and/or the other. The author worked with the great Rick Piper and Dave Garwood to bring this amazing but not fully finished FSX project to a more readily complete state for MSFS. It's a stunning model in its own right, and the devs chose, as a faster way to get it into the hands of sim pilots to temporarily slave it to the T-45 cockpit. No, the latter's glass cockpit is not the same as the T.2s, but it's a decent stand-in until a more permanent solution is found.
So yes, it's a freeware gift, and for those who have one or both of the payware Hawks, a true boon. The alternative would have been no T.2 for anyone until/if a fully bespoke cockpit could be modelled - easily the most complex part of FS addon development, particularly where glass cockpits are concerned. So it's a half full glass vs half empty paradigm, and not a hard one to grasp, in my view. For my part, I love the Hawk, but being careful with funds these days, sprung only for the T-45 as my first MSFS payware investment due to my interests in naval aviation. I've been tempted by the T.1, but am holding off for now.
Fortuitous for anyone who also only bought the T-45, you're in luck, because you don't need the Hawk T.1a if you're happy to use the T-45's sounds. For those of us familiar with FSX-era aircraft.cfg edits, it's literally seconds of work to do to get yourself sorted. I'm away for the day but will post the edits tonight on how to do that for those unfamiliar.