How did BA let you down?
You had a contract with them and you failed to fulfil your part of the contract by turning up at the airport on time.
You missed your flight.
Now whilst you can argue that it was due to circumstances beyond your control, likewise the traffic jams were not due to BAs actions or inactions were they? Purely and simply you failed to turn up on time to catch your flight.
Stop blaming BA.
A perfect summation of the situation.
Your only hope is that BA, somehow, develop a bit of compassion for your plight but I wouldn't hold my breath if I was you.