For our trip to London we took the train. In the past we travelled by rail but then switched to flying when budget airlines started to offer cheaper and quicker journeys.
But I'm not a fan of flying and we found out that the train is a good option these days. Firstly the journey time for the 400 mile route is four and a half hours. This is pretty amazing given that it made six stops en route. The plane takes an hour but once you add on get to and coming from airports, security, etc there's not much difference. Also the train is just a lot more civilised.
The Pendolino trains travel at 125 mph. A thing to remember is that, unlike many other European countries, the track is not a modern high speed line but one constructed in the 19th century. The Pendolino train can bank over like a motorcycle to provide comfort when cornering at high speed.
By booking well in advance the return journey was quite good value at £60. Flights can be cheaper but you have to add on a taxi or parking at Glasgow airport and a train journey from Stansted airport 30 miles north of London where the budget airlines fly to.