What would expanding Northolt Airport do to local residents?

Well if the existing runway was extended to match the length of those at Heathrow there wouldn't be many locals left in South Ruislip to complain about the pollution & increased noise.
Notholt's existing runway is only half the length of those at Heathrow, below is a mock up of the very minimum impact, if it was to go ahead this impact is likely to be far greater this however!

Put the mouse pointer over the pictures too switch to the current map view

Re-aligning the Runway has been mentioned in some reports which might mean something like this is being planned