Getting it marked as a "Gift" doesn't make the slightest difference to customs.
If its value is over £36 its still liable to import tax.
That's not quite correct.
When you buy something from the US (or any other non-EU country) it goes through customs at the Post Office unit at Mount Pleasant.
First they decide if duty is payable. The rate of this varies. However, if the duty charge is under £7, duty is waived. Also, a great many items are not dutiable. So far I have yet to pay duty on anything. Your experience may be different.
Secondly, they then charge you VAT at 17.5% on any item over £18 (not £36).
In my experience, items that are second hand, look cheap, are small, or under about £20 to £30 often pass through without any charge. But my experience is that they are less 'generous' than they used to be. Also, if duty was charged, you pay VAT on the duty (yes, even the tax is taxed!)
Finally, If any charges are due, the post office charge £8 (yes, £8 - its recently gone up!) for collecting the money. I'd be fascinated to know how much of this fee is profit.
So, if you buy an item for £20 (about $40), you might easily get a bill for £11.50 (£3.50 vat plus £8 handling)
I have to say I am now slightly wary about buying new items that cost £18 to £50 because of the handling fee. The VAT I can accept, but the handling fee is little short of daylight robbery. Once over £50 or so the savings on the UK price make it more worthwhile. In fact I have recently ordered an Elecraft K3 radio transceiver for around $2000 - I know I will get hit for maybe £200 VAT, but to buy something in the UK with similar performance would cost me far, far more.
I never ask sellers to make a false customs declaration. I don't want to seem a killjoy, but I think it's just plain wrong.