GB parental ratings not available to manually select

[qwerty12]

A little unorthodox but seems to work here on a show: in the metadata editor, you can right-click on 15 under the "Parental rating" heading and use your browser's "Inspect (Element)" on it. Expand <select is="emby-select" id="selectOfficialRating" label="Parental rating" class="emby-select-withcolor emby-select"> and change <option value="15">15</option> into <option value="GB-15">GB-15</option>. Select GB-15 and save.

I reckon that'd get a little tiring after a while, though. You might consider writing something utilising the Jellyfin API to mass-change anything with an OfficialRating of 15 to GB-15.