They can either get a taxi to Glasgow Airport and get on a plane which will probably be cheaper and less hassle.
Depends on your point of view. Personally, getting to and from the airports at either end (possibly a long way from where I actually want to be), having to be at the airport hours before the flight, checking in, getting through security, being restricted in what size water bottle you can carry on, checking luggage weight is within allowances, waiting at baggage reclaim, etc, etc, etc......it's a "no" from me.
Alternative plan: book the train, basically take as much stuff as I want/can physically carry, travel in all likelihood a much shorter distance to the station than to any airport, get on the train within a few minutes of arriving at the station, sit back and relax, pay a smaller mark-up for food and drink (or just take my own, again as much as I want), know that I'll ultimately be taxied to my final destination almost certainly with very little fuss in the event of a severe delay, also knowing that the delay repay process is also pretty straightforward....if you (or your relatives) still think flying is less hassle, fair enough - crack on.
Or they can get the Lowland sleeper and meet me there. They would get a taxi to Central.
Or, they could do the journey being suggested here which is get taxi from Matilda Road to Shields Road subway, get off at Buchanan Street, next to Queen Street, get train out to Dalmuir (7 stops?), get on the FW train to London to travel back through the place they have just come from?
Why would they do that when it is a lot more hassle and a lot more expensive?
If that's genuinely more expensive overall than the £25 you quoted earlier, then fine. I don't know what the taxi fare from your relatives' house to the nearest Subway station might be, and I'm not being funny with you here, but if that plus the Subway fare p/p into town plus the £3.70 p/p (assuming no Railcards) for the train from Queen St to Dalmuir comes to £25 or more, I'd be very surprised.
And it depends how much they want to accompany you on the train, I s'pose. Also depends on whether they care all that much about adding approximately 30-45 minutes to their overall journey, assuming they were planning on going into the centre of Glasgow to catch the Lowlander sleeper anyway, and bearing in mind that the journey back from Dalmuir to Glasgow before proceeding on down to London would be shared with you (which I assume they want), and which could be augmented with a hot meal and/or a cold drink (beer, whisky, whatever).
I am getting the FW sleeper regardless so can meet them there.
Knock yourself out, mate. Ultimately, it was just a suggestion. I'm fed up with your unnecessary hostility and rudeness. Enjoy the rest of your evening.