If you really, really like train travel, and would appreciate a longer, more scenic journey (and can guarantee daylight at the time of year), then there's also the possibility of using the Leeds-Settle-Carlisle line, one of the most impressive train journeys in England.

You can take the train (every half an hour) from London to Leeds, then change on to a rather rustic little train to Carlisle (every 2 hours) - see here for details. You can then pick up a Virgin or Trans-Pennine train (or, at a push, a ScotRail train via Kilmarnock - but I wouldn't recommend it) to Glasgow. Or the whole thing in reverse.