Changes

Get the Underground to the station Chiswick Park - from there you can find the entrance to the motorways M4 - its about 5 miles to the actual entrance to the M4, but you can walk along the main road for a mile and theres an Esso petrol station that is on the corner next to a bridge with the main london traffic running above you, and you can stand in a layby nearby with a sign saying M4 west, and someone will pick you up. Gunnersbury tube is also convenient.

 Heading for Cornwall, its much easier hitching the M4 and changing to the M5 than trying to hitch the M3.