I'm no expert but this is what I know...
There are two main network types that Pre phones around the world work on, GSM and CDMA.
In the UK there are only GSM networks (O2, Orange etc), here in the US, there are both GSM (AT&T, T-Mobile) and CDMA (Verizon, Sprint) networks. The GSM and CDMA networks also use different frequencies, so for example UK GSM phones may not work in the US and vice versa.
A GSM phone is also typically not compatible with a CDMA network and vice-versa (unless they are specifically designed to include both radios, which the Pre+ and Pre 2 are not).
If you have a SIM Card slot its a GSM phone. If you don't have a SIM Card its a CDMA phone.
The post I linked to above indicated that a Pre+ CDMA Com Board (Verizon) could be put into a Pre2, making it a Pre2 CDMA (Verizon) phone.
Given this I don't see why it wouldn't be possible to take the UK Pre+ GSM Com Board and put it into a Pre2 to make it a UK Pre2.
Hope that helps.