I hate spreading rumours so don't take this as if its true but, I was told by people who work at LNWR the last day was today, the driver of one of the 319s earlier also said it was today but that they'd be kept around for spares over the coming weeks. We'll see what happens I guess.
Tonight only saw three out 319219 319433, with 319220 on the 1639 London Euston to Milton Keynes Central rather full,
All week 319220 was with 319217.
However suspect at least for next fortnight or so, some 319s will appear, until 730 finally do appear, if not on those x3 319 diagrams, especially the 1624 London Euston to Tring one, for example, severe overcrowding will occur, especially using a solo 350, and on other services, where the 8 car 350 is now a four car 350, to accommodate a 350 unit vice 319.
It's possible end of month that will be it,
However, my opinion etc, the next fortnight or so, 319s, most likely out on Stanby, just like 317s did on Stansteads on Anglia last year. Personally I don't think, there is enough units to go around. Plus in service passenger ie the 730s, also need to be reliable.
Don't forget they could fail. Hence 319s on Stanby.