Sorry to be blunt....you took a risk and it backfired.
The rewards of ex-EU are brilliant, but do not always go without a hitch. In this case, unfortunately, it did not go to plan. These things happen, but I bet you are still financially better off that you would be if you went ex-UK.
Finally, why not just offload yourself at LHR and forget all the LHR-DUB-BRS hassle and extra expense? Seems a little silly to me.