I found the cabin noise in a 777 from Buenos Aires to Heathrow for 13 hours to be quite excessive. About 14 hours in an A340 from Auckland to Santiago was no where near as bad. I am guessing BA are using 777's on their new longest route from Heathrow to Santiago.
BA are using a 787 on the SCL route. It's a "long and thin" one as they say.
Terminal 5 is like a nightmarish version of the provincial shopping centre on Christmas eve - brightly lit, overcrowded, noisy and full of shops you don't want to visit. It's also one of the worst places to transit in Europe, very poorly designed and with multiple long queues. In fairness, B and C buildings are more pleasant. Terminal 2 is strange and small, nicer than terminal 5 but also too brightly lit and with quite disappointing facilities. My personal favourite is terminal 3, not so much for the terminal itself as for the fact that going there usually suggests an interesting destination.