Shed-on-a-pole, I completely understand your view point as a customer. You cannot deny BA's logic however. It's undoubtedly an unpopular decision with MAN/Domestic pax but difficult decisions such as these are made by airlines (and business in general) on a regular basis. With some notice (which in some situations can be achieved) MAN pax can make alternative arrangements. Perhaps refunded by BA? I don't know.
The fact of the matter is whilst LHR is constricted and does have problems, these are only occasional. You, and others, make it sound like it is every other week that LHR faces meltdown. IMO, you are just as likely to be snowstorm bound in JFK or PHL in the Winter.
I personally don't think LHR is that bad from my experiences. Sure, it's not as slick as other airports but i've never found it horrifying. Perhaps some people on this board simply resent LHR for what it is and what MAN isn't? And that comes from a big supporter and "lover" of MAN.