GB parental ratings not available to manually select

[chenks]

for my libraries i have the language set to "English" and country/region set to "United Kingdom"
for the vast majority this has matched the parental rating correct, in that the ratings all start with GB, eg GB-15, GB-PG etc.

however there is a handful of parental ratings where the rating is either missing or incorrect.
when trying to manually correct the rating none of the GB* ratings are available on the list to select from.

if the rating should be "GB-15", i only have the option of "15", and if i select that option it is different to "GB-15" meaning that i now have 2 separate 15 ratings in the filter list.

so how do i select the GB rating so that it matches all the other parental ratings?