Here is where you would start should you go to Kew and it certainly isn't encouraging:
Note particularly the following:
"The Friends of The National Archives will continue to describe this material from piece WO 121/137 onwards and the resulting data will
be uploaded into the system over the next few years."
Here is the description in the New Catalogue, which doesn't really go farther:
It does use the term "nominal roll' so it looks like film of a large collection of sheets for the year 1887 and it seems that each entry has the form of the one FMP sent you. What the "Friends" activity may add later is a mystery for now.
At best before start going off to Kew I would use their online help system which I think can get you to human at some level.
Hope this is of some help--- On Wed, 2/13/13, Éamann Ó Ruairc
From: Éamann Ó Ruairc <eamann@...>
Subject: [milgenire] Chelsea / Kilmainham pensioner
Received: Wednesday, February 13, 2013, 2:57 PM
I have at last found a trace of my GGF George Magee in the Findmypast records relating to soldiers retired with a pension.
Unfortunately the trace is limited to his name in an alphabetical list for July 1887. The reference is WO 121 / 257 / 1.
If I were to go to Kew, do you think I would find a file relating to his retirement?
Thanks in advance for any
The number “1789” after his name was his regiment number. I imagine that the letter “p” above his name stands for “pension”