Re 'seasonal'. At the risk of stating the (very) obvious, this is a scheduled airline. Granted there are summer and winter schedules, but what is winter in N Hemisphere.... you get the idea. Businessmen travel all year round, BA wants to make money by selling business seats to the USA and all round the world. You'd be surprised if you knew BA's most single profitable destination!
Having said that there are charter flights as well but these also go on all year round.