Relatively uninformed answer here, but they do use a handful of routes outside the normal timetable from time to time already. For example, Bradford Junction to Thingley junction, the Rhubarb Loop, and I think via an alternative route when Bridgend to Cardiff  normal route is not available. Also into Euston, and I think, and haven't I seen IETs from other operators getting to Eastleigh?

Westbury to Brighton presents various gauging issues to some trains - platform 3 at Salisbury, the tunnels at Southampton, and the Hamble / Swanwick bridge, and I suspect that the platforms at Brighton from the west would boo short.  I also know that some 3rd rail clearance issues have arisen.   They already go to Pembroke on summer Saturdays.  In passenger use, except for emergencies, the stations for stopping need to be programmed in too.

 
apart from the normal timetable for IET's,are they passed on other routes,

like beyond Westbury to Portsmouth and Brighton.

Milford haven and Pembroke

Worcester heading North