If MAN is clearly the UK airport of convenience for customers, then 5 x weekly is probably OK for the most part as I suggested. Where MAN is more likely to lose out is on those business travellers where MAN is more convenient, but LHR, or other options, are not inconvenient. In those instances, LHR with multiple daily flights and choice of timings will probably win out.
I guess the same holds for pax originating in or via SIN or IAH coming to the UK, provided the MAN option is sufficiently advertised and promoted by agents.