The post above from has a good link.

Click on the "NYC" link and look at the officer in the front right of the picture.

Note he has the three silver stars of a Chief Inspector on his shoulders and you can just abour make out the band of black embroidery on the peak of his cap. A suprintendent would look pretty much the same but the rank-markings on the shoulders would be a single crown and the peak detailing would be silver not black.

Not sure where this guy is from but, to be pedantic, the tunic design is *slightly* different for London's Met police and they don't wear the lapel badges but I doubt even a British audience would notice that. The thing that makes him stand out are his rank-markings and, for Superintendent especially, the most noticeable thing is the silver on the peak of tha cap - that's what makes it obvious that the character is a SENIOR-ranking police officer. If you're on stage, I guess that's all most of the audience will spot too - they won't be able to make out the exact details of the rank-markings on th shoulders.

Just a thought for you: if you can't get hold of a proper UK police tunic, don't panic. For day-to-day uniform they generally wear military-style navy-blue v-neck pullovers instead of the tunic. These have sholder and elbow reinforcement patches (also navy blue) and the epaulettes are attached to these. The uniform cap is still worn.

At a pinch, the US Navy black pullover would probably make a fair approximation if you needed it.