As a normal who formerly used train travel quite extensively, but, no longer does due to the abysmal service / unreliability and costs (even with a railcard) I did use the Standard Premium service last year. I couldn't book on line though as Avanti wanted me to open an account. No problem, I just went to my local station and booked an Advance rtn on the first possible date. However, this was booked by the usual uncooperative station staff who kindly, I discovered when I went home and checked the seating plan, booked me into the very Standard seats I had specifically asked not to be allocated.
Contrast and compare. On the day, I asked which carriage was for SP....a finger grudgingly pointed at one. Thereafter, when the guard came along, a quick swipe of the card, and £25 later, I enjoyed the ride to Glasgow Central...and then to Gourock.
Coming back, I "endured" the queue at Glasgow Central, a non event, station staff very helpful and communicative, and, repeat upgrade on board. Not interested in the fripperies and being fawned over in First, so yes, in this instance, the fare was reasonable and the train comfortable and on time.