In final unstressed position, true, ah represents the corresponding weakened vowel, ə, as in cheetah, hallelujah, loofah, messiah, rajah, Delilah, Jeddah, Sarah; but in stressed position and in monosyllables its “value” as ɑː is so self-evident that it is confidently used in respelling systems for native speakers of English.
Thus we Brits caricature Americans as saying Gahd instead of God; we represent Cockneys as saying dahn instead of down; and northerners think southerners say glahss instead of glass.
Yet there are two words with this spelling that constitute exceptions. One is Fahrenheit, which for most English people has æ rather than ɑː. The other is dahlia, the name of the flower, which in BrE is ˈdeɪliə. The OED (2nd ed.) gives the pronunciation of this word rather prissily as “ˈdeɪlɪə, properly ˈdɑːlɪə”. But DJ’s EPD (12th ed.) gives only ˈdeɪl-, and that is all I give for BrE in LPD. (Americans, on the other hand, may well say ˈdæl- or ˈdɑːl-.)
What I wonder is when, how, and why the eɪ pronunciation of dahlia became the norm in BrE. Carney says vaguely
this is derived from the name Dahl /dɑːl/, but with popular use has become anglicized…
The OED’s first citation is dated 1804, two centuries ago. If the name had been first introduced more recently, say since 1950, I think there is little doubt that we would all be saying ˈdɑːliə, “popular use” or not. OK, yah?