You will almost certainly have to purchase credits on it, to get full access for the necessary details, for subsequently ordering paper copies, if required.
As far as I recollect, the various Army records are more in the form of lists than conventional Statutory Registration documents.
I have found one relative recorded twice, first at the correct period of 1942, then 1947 when the records were being tidied up prior to Independence. On the same 1947 page, there were about half-a-dozen similar duplicates of other persons. This is unlikely in your case.
When logged in, select "Military" from the Menu line near the top.
This will produce a block of options headed-
Search all military records 1656-1994
The third line from foot-
Armed forces births 1761-1994
Click on that line, taking you to a Search Criteria page-
Name and approximate Year Range.
Carry on, following the instructions.
Assuming you find the entry, depending on its contents, you can then order up from GRO Southport by clicking on "Order Certificates" at top right of page.
Alternatively, you can capture the scanned image on screen, to a graphics file. --------------------------- There might also be other information about her father/parents on other parts of that site, worth checking.
--------------------------- The simplest and quickest way of paying is by conventional Credit or Debit Card.
If you haven't got such a card, check the ordering instructions.
There is a type of credit card available, akin to Gift Vouchers such as for books, which are thus virtually a "one-time only" transaction and therefore very safe; but you would have to locate a bank or such-like selling those.
Otherwise, you can order by conventional mail, cheque etc; but with greater delays. ==================================
If you have not already done so, also try Family Search-