Before the M25?
Of course, before the M25 was built Stansted airport didn't exist, or at least was tiny.

You'd have gone cross country, or to London and round the North/South circular. Both choices suck big time.

Forget the M4 - it goes nowhere near Salisbury. It is true that the long way round the M25 (via Dartford crossing) can be easier than the more obvious way, especially if you go via M3-A303 which is the best route to Salisbury. I went that way last Friday to get to Wimbledon and it was really easy - 10 minutes of queueing on the Dartford bridge and plain motorway cruising all the rest of the way.

Anahata