To close this out, called BA today to accept my fate.
Authorised £321 to be paid.

eTickets were issued, however I note I've been charged; Tax/Fee/Charge GBP61.40 + Amend fee GBP 70.00.

I am not asking any further question, my only hypothesis is that they priced the tax based on the two mexican legs and the EI flight.

Who knows!! Thanks for all your support and assistance.