LHR does need RWY #3, the gap is widening between it and AMS, FRA and CDG, those are it's current competitors. MAN is operating below capacity runway wise (what's capacity like terminal wise?). Traffic is the same as Dublin despite having far better capacity with 2 parallel runways as opposed to 2 intersecting runways that are only both used for a couple of hours in the mornings when winds allow in DUB.
A huge amount of Gatwick's traffic is only there because they can't get into LHR, so runway #3 at LHR would do huge damage to them.
Do you think MAN should be set up as the U.K. #2 airport and become LHR's main domestic competitor BAGSO? i'm not agreeing or disagreeing with you btw, just genuinely curious.
If LHR doesn't get runway #3, the UK as a whole will lose out financially, there is already a huge bleed of PAX to AMS from the UK regions heading east.