In about 2 weeks time, I need to get up to Glasgow from London for an important event that I can't be late for. I'll travel up the day before, but am aware Avanti are having some problems at the minute. Is this a poor choice of TOC, or route? Would flying or driving be a better choice, bearing in mind a limited budget? Thank you!
If you're travelling up the day before then Avanti will get you there, it just might not be on the train you hoped to catch!
To be fair to Avanti, the number of cancellations on the Glasgow services has reduced and so it does now bear some resemblance of a normal service (albeit still with the extended journey times due to Crewe stops being added to compensate for other cutbacks)
The most reliable rail option would likely be to take LNER (and optionally ScotRail) via Edinburgh, and perhaps surprisingly this can be competitive on both price and journey time.
Whether flying is a viable option depends on whether the timings and prices suit. Without knowing the exact dates and your requirements it's difficult to comment.
As for driving, how much that will cost you depends on what kind of vehicle you'll be using. But I would imagine that it would, at best, be comparable to the cost of the train - and it's rather a long way to drive, even if you allow most of a day.