Ignoring the change fee of £35 per person, the difference in taxes amounted to only £30.70 per person, making it a total of £61.40 for your new itinerary. I understand that this is for the more expensive departure tax (XD) and any other taxes or fees departing from MEX which may be slightly higher than from CUN.

I note that it appears you weren't charged the £70.40XO. For Avios redemptions like yours, even with fewer Avios needed than if flying with BA directly via LHR, the associated cost of taxes, fees and YQ make it less attractive for me. I am pleased you managed to sort it out in the end.