Think of it from a rationale corporate perspective (rather than cost cutting or customer service's perspective): it makes sense to have as few bases as possible, because it is easier to manage. Even if (as I imagine) South American based crew must have lower salaries than U.K. crew, there is still a cost-saving through simplification of the structure and logistics.

Ideally from the same perspective, you would only have one type of cabin crew as well, serving all London airports, not this EF, MF, WW, LGW and CF palaver.

While we all fear this rationalisation is purely done for cost-cutting purposes and will lead to further demise of the airline, it does not have to be - in this particular case there would be, if the airline wanted, plenty of excellent Portuguese speaking crew based in London.

I am not saying the airline will want to go invest in crew, but am at least trying to balance the argument without the emotional aspect that, quite naturally, the OP displays.