This is a nice example of the phonological function of the glottal stop in English, serving to distinguish words that might otherwise sound the same. The glottal stop, although consisting of no more than a silence, contrasts with its own absence. (OK, it may be more an effect on the phonation of the end of the vowel than just an instant of silence, but the point is the same.)
ˈstɑː bʌʔn = star buttonNot all English people use a glottal stop at the end of start before another consonant. Nevertheless it is unusual to have an exploded t. If it’s not glottal it’s likely to be assimilated to the following bilabial, producing an unexploded, indeed unreleased, p̚ thus ˈstɑːp̚ bʌʔn.
ˈstɑːʔ bʌʔn = start button
Worse, if he had been listening to an American rather than a Londoner, the ˈstɑːp̚ bʌʔn would have been not the start button but the stop button.
ˈstɑːp̚ bʌʔn = start button (BrE), stop button (AmE)Pity the poor EFL learner. It is difficult to discriminate auditorily between unreleased stops at the bilabial, alveolar, velar and glottal places.
But it's not just those learning English. Those of us who are not native speakers of Cantonese find it very hard to hear the difference between final p, t, k, all unreleased, in that language. (Hong Kong airport is called Chek Lap Kok, and each syllable ends in an unreleased/glottallized final consonant.)