Buy it unlocked from an Apple store only, not any other source. It will not work on LTE, but will work on 3G. Also, note that the warranty is valid in the USA only; you cannot get warranty service in the UK. And finally, unless you sneak it in you will have to pay VAT on entry, plus the sales tax on it in the USA, which varies from location to location, but will be between 5% and 10% of the purchase price. So you won't save as much as you think you will.
the A1429 (not available in NA) is the "rest of the world"iPhone 5
and the one available in Europe et al
It will work on 4G in UK but only on EE as they are the only carrier to
have the relevant spectrum
If you live in the UK and want to use an iPhone 5 in the UK, why do you not just buy an iPhone 5 from the UK, either directly from Apple and get an unlocked one or from a carrier of your choice?
I don't quite understand why you would want to purchase a phone from the US unlocked or not because you probably won't save any money on it after VAT and US sales tax and you will pay just the same for an iPhone model that you can then only use on 3G in the UK and that has no UK or EU warranty and that you will have to take back to the US if anything goes wrong with it.
If you want to use an iPhone 5 in the UK, then BUY it in the UK.
Quinny if you still can I suggest you buy the phone in the US as you will save around 15% because of the lower sales tax in the US.(Variable by state but still better than 20%)
You can also get it serviced in the Apple Stores in the UK without an issue and your warranty is still valid just like in the US. However, one caveat to this is if you get the AppleCare+ warranty you will be unable to use the accidental damage replacement they offer outside of the US but the regular part will still extend to two years.
Now the most important part is whether it will work or not. It will indeed work fine on 3G, I can confirm this as I bought my iPhone in Florida a few weeks back and am happily using it on O2. Now you did say that you don't really care about 4G, but just to clear it up. A North American(Model A1428 GSM) bought iPhone 5 will not work on 4G in the UK as EE(The only company offering 4G at this time) has a different band for it that only European(Model A1429 GSM) model handsets can use.
If you are further curious about the 4G bands they are bellow:
Model 1428 GSM: Bands 4 (AWS) and 17 (700b MHz)
Model 1429 GSM: Bands 1 (2100 MHz), 3 (1800 MHz) and 5 (850 MHz)
Just to further note EE and Three(4G coming this year):Use band 3(1800 MHz)
Hope this helps