The verdict? Traffic not good on Sunday afternoon into London, excellent departure from London on Wednesday evening when it seemed all the traffic lights were synced for us! There was a nasty bus accident on the M4 but otherwise the journey was as easy as by train and took about the same time door to door. No First Great Western to screw things up!
On the way up to London we stopped for lunch (a bowl of soup) at the National Trust's Stourhead property, something you cannot do by train. But the A303/M3 corridor is not the best route even though it is shorter in mileage. Parts of the A303 are still single lane each way, unbelievable after all these years! And the journey in to London from Sunbury Cross (the end of the M3) was tortuous, tiring and tedious.
The weather was fine on both journeys which undoubtedly made a difference. If it had been wet, foggy or icy we would have been better off in a train. But with fair weather and the chance to stretch our legs at Stourhead, the car was marginally better. But if a better train operating company had the train franchise, we would support it over the drive.