The BTP van siren I am sure is the common RSG universal siren (may be rebadged from another company for all I know), these are the controllers/sirens in our fleet
Universal Controller (CVS) (they make our lightbars too - we use the Stellar LED ones).
We only have wail and yelp installed on most of the cars however the newer ones have a seperate air horn switch (although it is a little odd to hear someone laying on the air horn thick and then a little astra emerge from traffic).
I can remember my driving instructor going "wail for the journey, yelp for the junctions, sod messing with the rest - two car convoy, both on seperate tones", not sure I agreed with entirely but that is probably why we don't have more tones installed, keeps you focused on the driving.