Yes so just to add if this is one booking you wouldn't have SDC. SDC is only for simple short haul returns and one way bookings. So I don't see that would be an option in your case.

Sorry, misread. Is the DUB to LHR a one way? In economy? Do you remember if you booked a basic or plus fare?

And to answer your question, no BA don't allow standby in that scenario. SDC is a confirmed change when done.