Quote: "B.A. want unrestricted access to a big airport with plenty of interlining capacity to European destinations, and only Silver-Boris can provide that."
No, Madrid-Barajas and Amsterdam-Schiphol could too, as could an expanded Heathrow. Madrid would also provide no competition for BA from a similar-sized carrier sharing it as hub (Iberia is owned by the same company), nor would an expanded Heathrow.
Not for second suggesting that any course of action apart from staying put at Heathrow is reasonable for BA.
Also not convinced that bankruptcy is an issue here, what would probably concern them more would be the loss of business if they upped-sticks to an estuary airport.