To find southbank, its basically on the river thames, just take a walk along there 10 minutes walk along the river bank from the london eye heading towards tower bridge, this is probably the most famous London spot.
theres also another spot just behind the london eye with steps etc, that appears in a few UK videos (blueprint etc), however not sure how to give directions from here. Beyond that you have the parks, guys at southbank could probably direct you to most pf them however.