The locale options are defined in glibc, which is developed by the GNU Project, and Manjaro imports glibc directly from Arch. Therefore, there is nothing that Manjaro proper can do about the availability of locales. 
But by the way, the UK locale does already feature a 24-hour clock, A1/A2/A3/A4/… paper sizes and the metric system, and so does the Australian locale. All you need to do is enable multiple localization options and tailor-pick what you want to use from either locale.
These below are my settings — I reside in the Flanders, Belgium, but I use US English as my language… 
[nx-74205:/dev/pts/3][/home/aragorn]
[aragorn] > locale
LANG=en_US.UTF-8
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC=en_US.UTF-8
LC_TIME=en_GB.UTF-8
LC_COLLATE=nl_BE.UTF-8
LC_MONETARY=nl_BE.UTF-8
LC_MESSAGES="en_US.UTF-8"
LC_PAPER=nl_BE.UTF-8
LC_NAME=nl_BE.UTF-8
LC_ADDRESS=nl_BE.UTF-8
LC_TELEPHONE=nl_BE.UTF-8
LC_MEASUREMENT=nl_BE.UTF-8
LC_IDENTIFICATION=nl_BE.UTF-8
LC_ALL=