I don't think UKARA would be needed, seeing as it's the buying (or receiver) who needs UKARA, not the seller (or sender).
It may need an orange tip (1cm+ bright orange on the end of the barrel, usually the whole flashhider) though, depending what state it's going to and where in that state.
And I think the receiver will have to pay the customs chargers, as you would if you bought a gun from a US website.