That's my point - whatever method you use you have to take a view on possible delays - if I MUST be somewhere I travel a lot earlier (a day extra for intercontinental trips) than if "it would be nice to arrive at..."
Driving to/from Cornwall can be a very pleasant experience - but it can also be the third circle of hell. Overall the train is best - but until they put in the Reading- LHR link connection it's not very good for connecting.
Hard to damn the air connection on those grounds IMHO