offering a higher standard of comfort than on the current franchised services
If that's their intended USP, and they can get paths on such a busy part of the network, I would have thought they would be better off with London-Portsmouth than London-Southampton. The latter as far as I know has services which are reliably worked by relatively high quality class 444s, whereas there have been complaints (fairly well-documented) from Portsmouth passengers about the use of class 450s (possibly even on fast services, can anyone confirm?)
The 444/450 argument on the Portsmouth line is not straightforward. There are plenty of 444s used to/from Portsmouth over the course of the day, about 50% of all trains, just not in the peaks in the tidal flow direction. It has never been as simple as the "no 450" pressure group like to make out. It hasn't ever been 444 fast and 450 slow either, the basic rule is that a 12 car 450 gives more capacity than 10 car 444, and that gives higher capacity than 8 car 450. There are also some 450s run through Southampton, mostly on Poole trains.
They won't get the paths though, so it's all a bit academic.