To reduce pollution near WHS, the simplest way would be to improve traffic flow through the Forest Hill junction; specifically, the A205 clockwise flow, i.e. in the direction from Catford to Dulwich.
The constant hill starts by traffic, including a large proportion of goods vehicles will be contributing a vast amount to local pollution. At peak traffic, vehicles may have four or more hill starts until they pass through the traffic lights.
I believe the current junction layout is a hangover from when trams ran though it as they required a large turning radius. The junction itself is too big, which greatly reduces the efficiency of each phase. The clockwise flow has two available lanes, but generally uses only one due to a lack of lane markings around the corner. The bus stops immediately before and after the junction also reduce traffic flow through the junction. The keep clear section at the exit of the station car park reduces the density of queueing vehicles near the entry to the junction and reduces the rate at which vehicles enter the junction.
It’s simply a bad junction that stalls traffic through Forest Hill, increases local pollution, and makes it more difficult for pedestrians to navigate around Forest Hill centre.
Here is an aerial show of the junction showing its excessive area: