As stated before, another LHR rwy isn't a sufficient important issue to unseat MPs, plus no decision would have been made by the election.
Not necessarily so. Remember the "save Kidderminster hospital" campaign a few years ago which lead to an independent, one issue candidate winning in the General election that year; also Martin Bell winning in Tatton, simply because the sitting MP was an (alleged) crook.
A well organised campaign can unseat the most firmly ensconced MP, even with our undemocratic first past the post electoral system - but I would concede that there is probably insufficient time between now and May to whip up voters into a frenzy over new runways at either LGW or LHR - that will be for 2020 (perhaps).