Apple does sell unlocked iPhones in the US and in many countries.
An unlocked iPhone will work on any supported GSM network world-wide.
The US unlocked iPhone is unlikely to work on any LTE network outside of the US or Canada due to LTE band support built into the device for the US.
And if you decide to buy the phone in the US, you need to be aware that the warranty will only be valid in the US and not the UK. If something happens to the phone you will need to bring it back to the US for service.