I agree with cws on completely changing the sector sequences.

For the DUB-LHR and LHR-DUB the only thing that would helps is if those flights were cancelled. The change of gateway b up to 300 miles used to apply regardless of which sector in the booking was cancelled, but over the last few months they have restricted it solely to the flight which has been cancelled. If DUB-LHR has been cancelled then you can change that within 300 miles, but otherwise expect to have to fly it.