Ruskie
You were right with your first post on this. If you feel you want to keep looking for another ribbon,
then may I offer these pointers? (If this looks too much like a bit of a lecture, feel free to ignore).
While the colours and pattern are of course important:
It has to be a campaign medal not a gallantry or similar as they would be worn in order before the campaign medal (in this case the GSM) he is already wearing.
It has to be one that is authorised for British troops to wear.
It has to be one awarded in the correct time frame (in this case 1962 to 2007 as indicated by the uniform pattern and the GSM).
Those parameters, which can best be checked out initially here
https://en.wikipedia.org/wiki/Military_awards_and_decorations_of_the_United_Kingdom#Order_of_wear confirm your proposal of the Oman Peace Medal which again I support.
Perhaps Pete would indicate what the purpose of the identification is? Is it to confirm other information held about a known individual or is it simply for interest? If it is the former then further investigation might be possible, with the latter then it doesn't really matter what the ribbon is does it?
maxD