CDMA is 3G. The Bionic is only CDMA when it's doing 3G.
The 4G in the Bionic is LTE, which is not yet deployed in the U.K. LTE uses a SIM. I had to install the SIM when I got my Bionic. Didn't you?
This thread is the first I've that the Bionic could do GSM. I don't know anything about it. If the hardware is there and just needs to be activated, then it's possible. One of the wonders of modern chipsets is that they often support multiple technologies so that the same chipset can be used in many different devices which creates better economies of scale for the chipset manufacturer.