In Cornwall for example a number of business travellers drive to Exeter and take the London train from there.
I've done this on several occasions, it is far cheaper and quicker to drive to Exeter or taunton and park at my relations, and get a train from there to anywhere else in the country.
As an example it takes approx 2 hours and ^15 quid of petrol each way to Taunton from my house near Hayle.
Looking at tomorrow morning as an example St Erth to Taunton quickest journey time is 3 hrs 14mins, average is 3hr 30.
ticket prices for peak time saves ^50-60 a return going from Taunton instead of St Erth at least, (last time i travelled, may have changed in around 6 months)