The use of the term "emblematic" isn't the best, due to its somewhat subjective definition. I'd personally go with "iconic" or "flagship" (which are still subjective in terms of stock selection, but less so in terms of definition).
The other issue also lies in the re-shaping of franchises over the years. While things like ECML and WCML are obvious (with Virgin and East Coast either being the original franchise holder, or retaining the same routes and stock to operate them, 390s and 91s respectively, although Virgin WC's iconic stock was probably 90s + Mk3 w. DVTs until the introduction of Pendolinos), franchises like Greater Anglia were once divided between Anglia Rail (86s + Mk2 w. DBSO), First Great Eastern (321s) and West Anglia Great Northern (PEP stock), with First Capital Connect emerging from Thameslink (319s) + WAGN (PEP stock). For the original Franchises, this would probably be easier than the new, more chopped around franchises, because that's when an appreciable amount of the stock was bought.
As such, I propose a new list based on the original TOCs.
GNER - Class 91 + Mk4.
Virgin WC - Class 90 + Mk3
Midland Mainline - HST
Great Western Trains - HST
Gatwick Express - Class 73 + Mk2 + Class 486
Virgin XC - Class 37 + Mk2
Anglia - Class 86 + Mk2
LTS Rail - Class 317 w. Green Stripe
Chiltern - Class 168
First Great Eastern - Class 321
Thames Trains - Class 165
Island Line - Class 483
Silverlink - Class 313
Connex SE - Class 465
Connex SC - Class 421
Thameslink - Class 319
WAGN - PEP Stock (both Class 313 and 315 used)
South West Trains - Class 442
North Western Trains - Class 142
Northern Spririt - Class 156
Scotrail - Class 150
Strathclyde Passenger Transport - Class 314
Wales & West - Class 158
Valley Lines - Class 143
Merseyrail - Class 508
Central Trains - Class 170
I feel these somewhat sit better than trying to come up with some for the current TOCs. Thoughts?