Originally Posted by
callum9999
That isn't UK tax - it's mostly a BA surcharge and probably includes airport fees.
You'll need to give a bit more information to get a definitive answer, but the best option I can see would be on Aer Lingus from Toronto - Dublin - Barcelona. The Dublin-Barcelona bit would be a bit cheaper on Ryanair (especially if you don't need checked baggage), but you would need to factor in long connections in case of delays (Ryanair have zero flexibility). If you'd be happy to spend 1+ nights in Dublin on the way there and back then buying tickets on Ryanair for the DUB-BCN legs will probably be better. (Being a penny-pincher, I'd personally opt for the Ryanair flights. Being an even bigger penny pincher I'd possibly go as far as to endure a megabus to Boston and save 15,000 avios!)
You'll have to call BA to work out the charges, but I'd estimate Toronto-Dublin to be 40,000 avios plus C$125 and Toronto-Dublin-Barcelona to be 55,000 avios plus C$200.
Thanks for the feedback. We'd be travelling with a baby, so long bus rides might not work for the others on the bus.
Do those BA charges apply to LHR incoming or outgoing, or both? We are thinking of trying to work in a stopover of a few days in London in one direction, unless it becomes crazy expensive.