Goods < £15 or Gift < £40. Anything declared more than that will attract VAT and clearance fee. (If you're lucky it might just slip-through, under the radar. But the RM are pretty hot on this these days, so probably not..)
I'd take the risk and buy from the US unless you need it quickly, you might get a bargain if it slips though, I'd guess you'll end up paying about £80 with the fees, VAT and import tax so nothing lost.
