If bmi cant make money on the LHR service there is something seriously wrong. Any fool can make money in LHR, It is the most pestigious, most sought after airport for any flag carrier. I just dont buy that domestic service is unprofitable, hubs like LHR exist to feed traffic from smaller destinations to larger ones. Perhaps the BAA should look at awarding slots for endangered/underserved services within the UK? like at some airports in the US. Or at least make BA & bmi's international slots a function of their non-trunk domestic ones.
Landing fees at LHR can be crippling, while the service may do well it wont do as well as a service with larger a/c to some long haul destination. They pride themselves on being the worlds busiest international airport and so price a lot of services out of the market
If what Northern Hero says is true though i dont think you have anything to worry about.