Layers. Wear lots of layers.
So you might wear a pair of tights, plus socks under your trousers.
Denim gets wet and doesn't dry quickly so don't pack just jeans to wear.
On top have a t shirt and a shirt and a jumper and a coat.
Don't forget gloves or mittens.
YOu will not believe how cold you can get if your feet get cold.
I wouldn't drive London to Cornwall, I'd get the train and hire a car there. There is no motorway in Cornwall so it can take ages to get anywhere.
By train it is about 5 hours (the same as driving if the weather is good). If you are travelling in first class you can pre book a meal in 'Pullman dining' this is proper silver service nice food, if you are in standard you can ask as you board the train and if there is room you can eat there.
There is also the option of a sleeper train, you get on in Paddington, go to your cabin (or you can just have a seat but that is no fun) and you will be waken up in the morning with breakfast brought to you.
Obviously you can just use the standard class seats and buy a sandwich.