I live in Renfrewshire where the only easy way of getting to Glasgow airport is by taxi, the nearest station is Johnstone which has frequent trains to central. I considered the sleeper as we are going on a Great Rail holiday on the Eurostar but it will be a fairly early departure and I wouldnt want to risk the sleeper running late, so we would travel to London the day before and Euston is only a short distance from St Pancras with a good choice of hotels. Coming back, I understand the Eurostar gets in mid evening, so we may take the sleeper then, though having read the Serco thread, I'm not holding my breath!
You are in a similar position to me as part of the parental unit live at Lochwinnoch and I think I have travelled between Mk and Glasgow by every conceivable means! I will say, clearly, train is my favourite option but it may not be the quickest
Coach: No, just no. Cheap but mind numbingly boring, slow and uncomfortable. And full of bloody students!
Car: Long, slow (ish) but this has been reduced with the opening of the M74/M8 link road. At least you get to choose were to stop when you drive yourself and for how long – Try Tebay services
Train 1: Direct via Crewe – I most often use the 06:23 when traveling the direct route which takes 4hrs:13 mins arriving into Glasgow at 10:36.
Train 2: Via Birmingham - These trains take about 5hrs4mins but are cheaper than above and often loess heavily loaded. NRES will suggest a change at somewhere like Lancaster onto the direct train but I tend jus to book the cheap fare and stay on the same train. The problem is that these trains MAY be a Voyager.
Train 3: Kings Cross – Glasgow. I have done this route in reverse and it is much longer but your get a proper train

It takes 5hr & 53mins
Sleeper: I find this a really good way to travel as it essentially gives me an extra day in Scotland and still has me back for work on Monday morning. It isn’t the most comfortable of nights sleep at times but I find it perfectly good, the lounge is nice and it is a relaxed way to travel. It is better southbound as you can have a shower etc at Euston. Price wise it isn’t always competitive but offers a good option for business travellers.
Plane: Luton – Glasgow is a quick flight, there is limited faffing around if you travel light and fares are ok. The issue for me is the journey to the airport as I don’t own a car. It means the bus form MK which takes 45-60 mins depending on traffic and either the bus to Glasgow or Paisley at the other end if I can’t rustle up a life.
I think the plane is quicker for me but it means I can’t work or really relax as you can on the train. Form London I am not so sure which would be quicker by the time you add the journey to the airport, parking etc and the journey to and from the city centre if you fly into Heathrow or Gatwick. It must be close. I do wonder if it is quicker to fly from Glasgow to Luton and train it to London from there.