You need to apply to the train company directly for inter-personal season tickets, but it is for businesses really. The FRPP entry doesn't appear to have been updated properly in a while, it lists....
1.5 times the normal fare
ATW (Arriva Trains Wales)
FCC (First Capital Connect) (sub note to say the pricing manager can change this)
FSR (First Scotrail)
MML (Midland Mainline, now East Midlands Trains)
NCH (???)
NIW (???)
NT (Northern Rail)
PSC (???)
PSY (???)
PTW (???)
PWY (???)
SWT (South West Trains)
VWC (Virgin West Coast)
VXC (Virgin Cross Country, now Crosscountry)
Apply to the TOC (no price data)
GWR (First Great Western)
GXR (???)
PMR (???)
RCL (???)
RMS (???)
SET (South Eastern Trains)
Apply to TOC (no price data, Annual tickets only)
SN (Southern)
No interpersonal tickets
c2c (c2c)
EC (East Coast)
LER (???)
STS (???)