Did you try the Canadian BA line? Not sure if it makes a difference, but they changed mine to YUL-LHR-DUB without any issue. I was prepared for a fight and had the flights all ready to tell them exactly what I wanted, but the rep booked me on them before I could tell her they were the ones I wanted. You may want to mention EU261 and what your options are under the regulations.
If all else fails, file a claim under EU261. I would expect that they would respond pretty quickly rather than deal with the regulators