I've had the same problem as the OP regarding the inability to switch off LTE & 3G on the iPhone 5, using the EE network in the UK.
After accepting the last carrier update on the phone itself, the 'Enable 3G' menu became 'Enable LTE'. When this option is disabled, the data connection defaults to 3G, whether a 2G (GPRS) signal is available or not. Prior to the carrier update, it was possible to use the less battery-intensive 2G connection for data, when I chose to use it.
I spoke to AppleCare about this & I was told that the carrier updates are in the hands of EE UK. AppleCare are unable to let me overwrite the carrier update with a previous version & EE UK can't/won't do it either. Having contacted EE UK, they told me the only way to switch off an available 3G data connection was to switch off data completely, within the cellular menu.
This inability to manually switch off an LTE & 3G data connection on my iPhone 5 means that its battery will be depleted unnecessarily sooner than I'd like, but I no longer have any control over that. Had I known that the carrier update would have had this effect, I'd never have installed it.
The same problem applies to all of the carriers making use of the EE mobile network, including T-Mobile, Orange, Virgin Mobile, Vectone & Ikea Family Mobile, when used with the iPhone 5, as the current carrier update is likely to be EE 13.2 for all of them.
It'd be useful if users in this same position could downgrade their carrier updates themselves, but this doesn't seem to be currently possible. :(