I'd avoid the Dartford Crossing like the plague, it's always nose to tail in the afternoons, so anti-clockwise is the best of a bad bunch. As was said earlier DONT take the M4 - it's awful - take the M3 until after Basingstoke, then the A303, then near Andover look for the signs for Salisbury - the A343 I think. Not a pleasant drive until after Andover really. I don't envy you the trip.