@John Smith 19
Good point about the cost/benefit analysis - no, but that's probably because those aren't generally visible from the supplier's side. But the tender process for most of these type of systems are typically bragged about for ages in the press by either the home office, NPIA or the forces, so you can generally put two and two together. To be fair, most of them aren't "must have new tech" decisions, because the govt wouldn't sign them off. Having said that, there does seem to be a degree of oneupmanship between forces, so an element of that creeps in later.
The real problem with forces going in different directions is that the IT body "representing them" (was PITO, now NPIA, next God-knows-who) can recommend, but ultimately has no teeth. Every time someone gets close to giving them the ability to mandate purchasing, a bunch of Chief Constables kick back saying that their force needs something different because of the way they work, yada yada yada.
Since Soham->Bichard, the appearance of the PND gives NPIA a carrot to use on the forces instead of a stick. Since the forces *have* to work with PND, they start moving in the direction of common data formats, at least for centralised intelligence. However, for all other systems (Custody, Case Prep, Command & Control etc) the bets are still off.
Re: your last paragraph - no, only a small subset of the forces' info is sent to PNC/PND, and that's only at the will of the local coppers. Everyone still has their own datasets and they're all fiercely protective.
The only things that will change the way the police buy their IT are a real effort by government to align the way forces do their day-to-day work, and some serious investment (both money and time) in the tendering process (instead of just handing it to some bored fifty-something coppers on the way out to retirement).