Re the boost engine, as mentioned above, I was given the impression that these engines were removed and I'm sure I've seen this in print. As the guys above have said, they obviously weren't but I'd be interested to know just how often these engines were used in service and how reliable they were.
As I said, I had many Trident flights, a good number with time in the cockpit and saw hundreds of Trident 3 take offs in the 1970s/80s and can only remember a handful of times when the engine was used.