It seems like the OP is doing an ex-EU from DUB but wants to "throw away" the final LGW-DUB sector. If/When it comes to re-booking just choose an overnight, but less than 24 hours (to avoid APD) LHR-DUB that way your bags will not be checked to your final destination, or choose a LGW/LCY-DUB flight. The agent might think it slightly odd, but should be allowed.