In these circumstances, the claim is usually dealt with retrospectively as when a relative/loved one dies, the bereaved will rarely have all the paperwork available as other things take priority over changing airline tickets and faxing off a copy of the death certificate or whatever is not practical given the timescale such events entail.

As for the OP, I'd be interested to know what type of fare he had. I would have thought that most YYZ-DXB fares would have a degree of flexibility. I can only imagine that the fare was perhaps broken at LHR, meaning an ex-LON I/D class fare put a spanner in the works. Would be bad luck if this had happened as the saving would likely have been minimal as compared to the lowest throughfare YYZ-DXB.