They actually did that once.

Or, specifically, when -[UIApplication openUrl:] was invoked with a tel url, version 3.0.0 would take you to the phone app, but not actually make the call until the big button was pushed. Versions previous to that would autostart the call, and versions after that would autostart the call.

I don't remember the reason why Apple changed back to autostarting the call. I guess it was because of all the autodialer apps meant to make it possible to one-button call from the home screen. People finding the phone not actually making the call might have considered this next layer of required user interaction a bug.

In the US, premium-rate is often known as 900 or 976 due to those being the area code and local prefixes, respectively. So in that area, I suppose the dialer could prompt before calling with a simple check. Is such a filter possible with UK premium rate numbers?