Dear OP, the fallacy in your argument is that BA isn't the proprietor of Terminal 5.

Heathrow Airport Limited owns and runs the airport. BA pays rent on some office space, rest areas and lounges in the airport, but it's all owned by HAL. The only "things" that BA owns outright are IT assets at gates, and I'm not even sure about the cabling there.

One leaseholder can act on its own turf, but the overall mice/rat infestation goes beyond that. The problem needs addressing by HAL, as the nesting areas and so on go deep in the terminals (and let me tell you, ALL terminals are affected, T3 more than others).

If Heathrow were a State-owned asset and HAL its managing company then there'd be some incentive, on their part, to actually do something about it, but unfortunately HAL answers only to HAL's investors, and what these want is returns, not less rats. Airlines can complain until they're blue in the face, but as it always happens the balance of power isn't on their side: they need HAL more than HAL needs them. It's never said in as blunt terms as I'm about to say, but if airlines don't like what HAL does, they can always leave and find another airport. Over the years, at the AOC meetings, there've been a few airlines (especially state-owned relative newcomers) who arrive ready to show HAL who's boss, only to retreat tail firmly tucked between their legs.