Hell, if you can afford to pay for and run an AW139, the salary for a second pilot is peanuts. I am not sure, but I suspect that under JAR/EASA rules a second pilot will be mandatory. Basically the UK CAA follow EASA/JAA for state aircraft.
What ought to happen is that a police force or Consortium decides what it wants do with its Air Support. That requirement then enables you to define what sort of aircraft to buy.
In reality, I have yet to see it done that way at all.
An AW139 would give you all sorts of extra capability: whether the police need that is another question.
For a place like D-P, if they had the extra money and the need, the best thing to do could well be to have 2 helicopters based in different parts of their patch, because of its size.
But there are all sorts of other ways to spend more money - you could start by extending operating hours with your existing aircraft, which would be an option for most ASUs.
At the moment though, most ASUs are struggling to maintain their existing budget, never mind get a major increase.
It feels like bad timing to me. A bit like the marketing of the EH101 with its 30+ pax capacity to the offshore market. It became available just at the time that the number of people working on an offshore platform reduced markedly. I am open to correction, but I believe it nver sold any into that market.