Shytorque -
Because when 121.5 is constantly buzzing with non-emergency transmissions I for one am guilty of deselecting it to give my ears a rest and forgetting to reselect.
So the problem isn't really with those using the frequency correctly... why not consider getting something put in your company SOPs?
I'm single pilot these days and so I have to manage both radios all by myself but don't have a problem with remembering to adjust volume levels. I'm constantly using box two for something or other, in addition to my main comms on box one, and therefore constantly checking the squelch and volumes. I often listen to the PP / TF calls out of interest, provided they aren't blocking something on box 1.
As far as usage of 121.5 in the Gulf is concerned; are UK pilots' transmissions reaching that far afield and causing a problem?