startford? me no think there's a place called that. guessing you mean dartford? anyway. you follow the signs for the M25. you then go clockwise round this motorway (it's circular) - towards Dover/Kent/South. get off at junction 7 (M23) and follow the signs for the airport, which is about 13 miles down the M23...you come off at exit 9 and take the A23 but by this stage it'll be obvious
From Stratford, take the A102 Blackwall Tunnel road, which automatically leads on to the A2. From the A2 get on the M25 and then follow the directions given by rja211077.
You've obviously never been to East London rja. It is a jolly wheeze confusing all the visiting Americans when they ask where the theatre is. Then you have to tell them thats Stratford in Warwickshire!