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"We are thinking of trying to work in a stopover of a few days in London in one direction, unless it becomes crazy expensive."

If you are willing to get yourself from DUB to BHD (Belfast City Airport), you can get a one-way BHD-LON-BCN as a Reward Flight Saver for 7,500 Avios + $27 per person, with a free stopover in LON.

If you want to go DUB-LON-BCN with a LON stopover, that would cost you 4,500 Avios + $27 on BA, plus 7,500 Avios + $27 on BA again, as two Reward Flight Savers.

The DUB-BCN nonstop on EI would cost 7,500 Avios and most likely a higher cash co-pay, if you can find award space.

Also, do you plan to get a separate seat for your baby? If not, be aware that lap infants are typically not free on international flights.