It's been added. However, please bear in mind that Duo's official learning language is American English. We do get reports about us not having some British words added. We do try to add as many as we can (not only British, but Canadian and Australian too), if there is no potential confusion in context, to be fair and please as many learners as we can. But of course that are still some that we haven't added just yet, since the course originally included American ones only.
Thank you for your comment and patience. ^.^