Everything that affects potential profitability feeds into that forecast, including how Brexit might alter the market.

No one can dispute that.
However the slots were sold off long before brexit was a gleam in anyone's eye. The residue of the company was a stripped out husk losing money from that time if my understanding is correct. From comments I have seen elsewhere it was a dead duck a long time back.
"The airline was heavily loss-making, and no-one was prepared to put in extra funding. The average passenger load per flight was just 18, meaning even the small regional jet planes flown by Flybmi were less than half-full, The typical Ryanair departure has 10 times as many passengers.

Since Flybmi split away from BMI, investors have pumped in £40m, which represents a subsidy of about £13 per passenger ever flown."