GSM and CDMA are two competing wireless network designs. GSM is used globally. CDMA is used primarily in North America (where GSM is also used). Since you live in the UK, CDMA is a non-player. The 8130 will not work in the UK. You can't "unlock it" to work on O2, since it is not a GSM device (and no SIM card, either).

The 8120 is the Pearl2 for GSM networks. The 8130 is the Pearl2 for CDMA networks.

The 8120 is so new that it is not part of the BlackBerry Comparison program on RIM's web site.