Was it really the 22nd Feb 1985 that I last flew Zulu Kilo, back from EDI (and I gave the landing away)?
Back to the question, the Boost engine just had a small switch to control it. T/O and climb if I remember correctly, with a gauge showing the thrust.
Where was it? Well, starting from the top of the throttles (in the photo mentioned earlier) move to the right and the first gauge you come to is the Droop. Next to it, and slightly below is the boost thrust gauge with the control below the gauge.