Except for few devices sold by T-Mobile for a limited period, I never saw SIM- or netlocked phones sold with postpaid contracts.
And regarding the VoIP-ban, you may have missed
O2's announcment this week, according to which they will officially allow VoIP without any surcharge.
However I agree with you, when it comes to the preference of prepaid over postpaid. Postpaid was interesting in the past due to the highly subsidized handset prices, but those subsidies have decreased significantly, so it doesn't pay off to bond yourself two years to the mostly unattractive postpaid tariffs.