The concept of prestige based on loco is just for us rail fans. Each of us would consider it as a prestige if our favorite train is hauled by our favorite loco. For IR, all locos are the same and allotment is always done based on operational feasibility. The authorities have nothing to gain by not allowing a particular train to run with a particular loco.
For example, as you pointed out that all Raj trains seem to be getting P7 as a standard link, it's not because IR feels a P7 is a 'prestigious link'. The reason is that LHB rakes require more powerful compressors in the locomotive to be able to operate the brakes and other equipment. The 3 phase...
more... locos like P5 and P7 had these higher power compressors right from the beginning. The older generation of WAP4 locos did not have sufficiently powerful compressors for this. That's why, when the Raj's stopped getting WAP5 locos, they were changed to use WAP7 almost exclusively, except for shorter rakes and less aggressive runs even though the P4 was powerful enough to maintain the schedules. Over time, many P4 locos were upgraded with new equipment to handle this problem ( like how SK always runs with WAP4 even though it has an LHB rake) and are now being used for LHB rakes more.
Anyways, I don't mean to imply that BBS Raj does not 'deserve' a P7, I just mean to say that all the locos are the same for IR. They'll use whichever loco gets the job done.
My apologies for the long chain of arguments. :(