This tutorial shows you how to say "a bottle of water" in 4 different London accents. The moral of the video is that it is your choice quite often whether you say a sound or not and British accents are pretty flexible these days. For example, if you are speaking multicultural London English or Estuary English then the choice is yours whether you drop the /t/, or actually, replace the /t/ with a glottal stop sound, which has become THE sound in modern British English.
