I haven't ordered any certificates from the "Military" records, as what appeared on-line was sufficient for my requirements.
I've taken a look at that Index information-
Pixton Anne Ceylon 1860-67 127
I have just followed through on to the GRO site.
From what I have seen, I think you would be better to order by conventional mail and cheque etc, although this will lengthen the time taken.
Try phoning first.
The problem is that the GRO Input page is designed for conventional BMD Certificate of the English and Welsh Statutory Registration type, so doesn't readily accomodate the less formal nature of those Overseas Military records.
Eventually found the ordering information- " How to order overseas certificates Applications for certificates of overseas birth, death or marriage certificates can be made in a number of ways:
by a personal visit to the Family Records Centre in Islington, London
by secure online ordering
by calling +44 (0) 845 603 7788, giving details of the birth death or marriage and paying with a credit or debit card
by post to the following address, giving as much information as you can about the birth, death or marriage: