Hey, even though I'm not a native (British English) speaker -- I'm American -- but I do know the literal meaning. I think.
"..There comes a time/A flaw that breaks/This very smug mug of mine..." I think this describes the moment when a (serious) flaw in the other person causes the narrator's (in this case, Morrissey's, if he's singing autobiographically, which he apparently does) face to 'fall', as in not be so satisfied or secure as he once was, presumably because of how horrible the flaw is...
"...This dawn raid/Soon put paid to all the things/I whispered to you at
night time.."
I take this to mean that the dawn raid (something traumatic/eventful/important that happened early in the morning) -- which in this case, was something very bad -- caused all of the 'nighttime whisperings' (tender things, most likely) to come to an end. ("Put paid to" is a chiefly British saying which means to come to an end, or finish.)
It's the most beautiful song Morrissey has ever written in my opinion. Ah. A true masterpiece.
I hope this helps! Do you want my interpretation, also?