Solved, I'd missed something quite obvious.. thanks
The layer itself is very very simple:
layerinfo "type" = "i18nc";
layerinfo "name" = "British English";
layerinfo "redist_uniq" = "i18nc/en_GB1";
layerinfo "langcode" = "en_GB";
layerinfo "author_name" = "Smigs";
layerinfo "author_email" = "smigs@livejournal.com";
layerinfo "source_viewable" = 1;
set lang_current = "en_GB";
set lang_fmt_date_short = "%%d%%/%%m%%/%%yy%%";
set lang_fmt_date_med = "%%dayord%% %%mon%%, %%yyyy%%";
set lang_fmt_date_med_day = "%%da%%, %%dayord%% %%mon%%. %%yyyy%%";
set lang_fmt_date_long = "%%dayord%% %%month%% %%yyyy%%";
set lang_fmt_date_long_day = "%%day%%, the %%dayord%% %%month%% %%yyyy%%";
set lang_fmt_time_short = "%%HH%%:%%min%%";
set lang_fmt_month_short = "%%mm%%/%%yy%%";
set lang_fmt_month_med = "%%mon%% %%yyyy%%";
set lang_fmt_month_long = "%%month%% %%yyyy%%";
Obviously my intention here is to ensure correct date formats across any and all styles I choose to use.
If any other British English speakers want to point out corrections or additions, please do - some of the chosen formats are more my personal preference than anything else. ;)