With LHR being close to capacity it makes sense to pro-actively cancel flights, indeed the airport authorities encourage them to do so.
Greater spacing, greater numbers of go-arounds, extensive holding and possible diverts all conspire to make operating a full schedule very difficult.
Pro-actively cancelling flights, however understandably frustrating that can be, is the right thing to do. It gives everyone (including the airline) the chance to make alternative arrangements to try and get people to where they need/want to be.