I've bought a few bits from the States over the years. Current thinking, is a bit muddled, because I still think it's 'pot luck' but recent advice I was given, suggested that the Revenue don't get interested under a certain limit - recently stated to be £100 value.
I'm not sure that's true. I recently brought a 'V' twin exhaust via Ebay in the States & was sure it would get "hit". It didn't, but, the seller had marked it up a a "Gift" on the import papers - without me asking him too. My understanding was that if it was marked as a gift it didn't incur "duty" but the same sources, suggested that if it was over the £100 limit, it wouldn't matter if it was marked as a gift or not, it would incur duty. I know that over the years I have had items charged that were well under £100 value, but that was few years ago.
My first advice would be to tell the seller about our wicked Customs men & see what they say re. ticking the gift box

I did once ask a lady selling thread on behalf of my wife if she would mark a parcel as a "Gift" & she went off on one saying it was a Federal Offence, to do it, or to ask for it to be done. The 'G' Men haven't showed yet.