After a previous experience with British Air four years ago, in which they refused to make an announcement regarding my peanut allergy, and gave us the option to disembark the aircraft, my faith in BA has been restored!
As I boarded my 11h30m flight to LAX in August 2018, I told three different flight attendants about my peanut allergy, and each were extremely helpful. The crew made an announcement before take off once everyone had boarded and the flight attendant reiterated my allergy to the people surrounding my row. They checked all the meal ingredients and checked on me at regular intervals to ensure I was okay!
The return leg was also an extremely pleasant experience, the crew made three announcements, one just after I informed them, one whilst people were boarding and a final one just before we took off.
Alongside EasyJet, British Airways are one of the best airlines for flying with a peanut allergy (especially long-haul) and they will do everything in their power to ensure you are safe throughout your flight.