The policy with most airline about flight delay compensation is the delay has to be over 3 hours. When submit claim online BA.com, you need gather all the proofs including upload boarding pass, provide flight no., to and from, details about delay, and maybe a screen shot of the departure time shown on board at airport or the airport website (all the flights departure time) under heathrow.com (for example).

However, BA seems to be very tricky and in my case, I could not claim the delay even though it is more than 4 hrs. What happened was while my flight was delay from PISA to LHR due to weather condition, all passengers were boarded on plane under 3hrs delay (2.75hrs from original departure time on ticket). The BA flight would not leave, just make all passengers wait inside the flight for another hour or longer. There were no food or drink served during the wait either. I had thought I was legit to be able to claim the flight delay because in realtiy it exceeded 4 hrs delay. However, it is all about proof. After landed LHR, I went on the BA website and the departure from PISA airport showed delay 2.75hrs (under 3hrs), not including the 1 hour waited inside the flight. That was tricky part. BA knows they would have to payout a lot lot of money to compensate over 200 passengers if the board shows > 3hrs departure.

So keep in mind the claim process all needs documentation. I think baggage delay over 12 hrs claim is much easier because BA cannot control courier delivery time as it is from outside companies. It is much easier to claim baggage loss or delay.

Hope this helps.