NYPD does perform surge drills using lights & sirens (such as http://youtube.com/watch?v=lQp97jIff2c ), which is something I can’t see how you would test without doing so…since a key point is controlling the streets with the surging units.

It is highly unusual, along the lines of using lights & sirens on move-up assignments. Usually you don’t need them, but there is some move-ups where public safety would be impacted too greatly “proceeding with traffic.” Even in my suburban / rural area I have seen situations where traffic has been gridlocked and it would take one to two hours to approach special events without using lights and sirens. Likewise, without commenting specifically on the U.K. situation, I’m sure there are a very few training activities that can’t be properly emulated without resorting to using lights and sirens.