I'll echo what others have said re: reliability and running the risk of missing your flight, but on top of all that, this train is notorious for being one of the most overcrowded in the country, so even if it's on time, you're likely to have a pretty lousy journey.
Its only overcrowded from Wigan/Preston to Mancehster Piccadilly. Boarding at Glasgow, alighting at the Airport should be stress-free.
...But needless to say, I'd strongly support travelling the night before for this one. Saves stress and a ludicrously early alarm.