Even if the runway ever gets built (
) I do not believe that much effort would be put into using it to serve UK destinations. The prestige of airlines from all over the world operating big airliners into LHR will always beat regional jets and, let's be honest, the airport will earn more revenue from bigger aircraft and the greater number of passengers they carry.
National interest will not enter into it, unless it is that blinkered Londoncentric national interest alluded to earlier.