If you are looking to travel from Cornwall to any London airports, then flying is probably not only the quickest, but probably the cheapest way.
RyanAir fly once a day (currently 7pm) to Stansted airport,and it costs from £10.
Air South West fly several times a day to Gatwick airport, prices from £40.
British Airways resumed flying to Gatwick in 2007.
Check TrainLine for rail prices, but expect to pay at least £100.