Aside from the pollution/CO2 dimension, I do see a substantial difference between a 3rd runway at SIN and LHR. Unless the geography has changed dramatically since I lived/worked there, Changi is on the eastern end of Singapore Island, with the runways orientated north/south (02/20 in my days there), putting approaches either over the sea or underpopulated areas of Malaysia. LHR is just west of Greater London, with approaches to 27 over densely populated areas.
These differences might account, in a general sense, for the respective populations’ opinions on a 3rd runway.