These are my best estimates for the HST passenger capacities:
East Coast HST (2+9): 431 standard class, 113 first class.
East Midlands HST (2+8): 357 standard class, 112 first class.
FGW long distance HST (2+8); 375 standard class, 112 or 118 first class.*
FGW high density HST (2+8): 461 standard class, 96 first class.**
Crosscountry HST (2+7): 389 standard class, 70 first class.
Grand Central HST (2+5): 221 standard class, 48 first class.***
* First class capacity dependent on buffet type used.
** These sets were previously in 2+7 formation, but have recently had an additional standard class carriage added, converted from buffet vehicles. I am not sure of the seating capacity of these new carriages, but I presume that they are similar to the existing high density vehicles.
*** Grand Central also operate 2+6 HST formations, but these vary in formation and often use declassified first class carriages as standard class accomodation.
As built, Intercity 125 first class carriages seated 48 passengers and the standard class carriages seated 72. Following the first Intercity refurbishment started in 1984, the capacity of the standard class vehicles was increased to 76 through the addition of additional airline style seating.
The first batch of HSTs, for the GWML, were delivered in 2+7 formation with two first class vehicles (TF), three standard class vehicles (TS), a TRUK and TRSB, as prior to introduction of the trains it was felt that separate catering facilities would be necessary for first and standard class. The TRUKs seated 24 passengers and the TRSBs seated 35. TRUBs were introduced later on in the build, that seated 17 following the 1983/4 refurbishment and became TRFBs.
However the requirement for two catering vehicles was soon dropped and by the time of the first refurbishment, by which time the TGS vehicles, seating 65, had been introduced, the formations had settled down to be as follows:
Crosscountry (2+7): TF + TRSB + TS + TS +TS +TS + TGS (404 standard class, 48 first class)
Western Region (2+7): TF + TF + TRFB + TS + TS +TS +TGS (293 standard class, 113 first class
Eastern Region (2+8): TF + TF + TRFB + TS + TS + TS + TS +TGS (369 standard class, 113 first class)
The Western Region sets were lengthened to 2+8 formation during BR days, following the electrification of the ECML, bringing them up to the same passenger capacity as the Eastern Region sets.