The Trident 3 was distinctively noisy with the boost engine generating a higher pitch more military sound on top of the already noisy Speys but I don't think it could touch the VC10 for sheer noise.
I also understood that as well as no controllable thrust there wasn't alot of engine monitoring going on for the booster having once heard a Trident pilot ask the company aircraft next to him at the holding point if he could see if the inlet doors were open for the forth engine.
As for the corporation rivalry at the time growing up in Stanwell it virtually decided who your childhood friends were if parents worked for 'the other lot' . Not many neutrals as back then Stanwell was virtually a company town for BEA BOAC , but I dont think it went as far as denying subload trips. In the early 70s I did a lot of these on my fathers BEA concessions often accompanied by a friend who was a BOAC FO and at times by others from the dark blue side.