All that would happen if Boris Island was built and LHR was closed is that BRS would gain another feeder route (BA). I can't envisage any new LH starting. Maybe if we had another 1000ft of runway to play with, but I just don't think the economics stack up for EY/EK/QR (even with the 787, maybe things will be different with the A350?) due to performance constraints.
RE the cancellation of the TOM long haul services, I thought it all happened after the report that was issued regarding the hard landing on 09, which lead to some serious airframe damage a couple of years ago? Basically the runway is seen as marginal for operating such services due to its undulating surface and length.