The CDMA versions are permanently locked to their respective carriers (Verizon and Sprint) and can not be used with any other carrier when in the US. It is possible for the carriers to unlock the SIM slot for use with foreign carriers when roaming internationally. They will only do this for customers in good standing.
It appears that you would be able to get LTE coverage while in the UK based on what you posted.
Kep in mind though, that the CDMA phones will not support simultaneous voice and cellular data access on the networks in the US due to their single radio design.
You might be better served to purchase an unlocked iPhone 5 in the UK. If that one will not use the AT&T LTE network, at least it would be able to access their HSPA+ network, which is substantially faster than 3G and allows for simultaneous voice and data.
The UK LTE can only use a A1429 GSM and the only carrier to be able to provide
the suitable spectrum is EE (Orange and T-mobile)
So in effect the iPhone 5 will only provide LTE on EE networks in the UK .
and the UK iP5 model is the A1429 GSM