The problem at LHR is gate availability and stand space. When the airport gets screwed by the weather there is only a finite number of movements that can be made, and because BA has such a huge presence at LHR something has to give. The BA rationale is that it is easier for people to travel by other means within mainland UK than in Europe or the rest of the world as there is no sea crossing to be made within mainland UK. Also if BA can send a plane off to eg Barcelona etc they are pretty sure its gonna get there, if they send one off to Glasgow or Edinburgh they're not that sure its gonna get in and back out if the weather depreciates.
BA policy is to stop domestic routes when weather dictates as it is the best option of a bad lot. Yes I know it can mess things up for a lot of people but you cant please everyone all of the time and they really do try and keep disruption to a minimum as weather delays and cancelations cost them a heck of a lot of money and thats what it boils down to folks.