I think I may have cracked how to avoid customs duties, at any rate as far as coins from the USA are concerned.
I received a coin today, which I bought from e bay on 4th February. When buying the coin, there were two international postage options given - one massively cheaper than the other. Naturally, the more expensive option might seem the obvious solution, especially if it'a a valuable item. But in fact delivery date expectation is the same, and they are both very adequately tracked online, both on the USPS website, and then on the Royal Mail tracking site once it arrives in the UK.
The difference between the two types is that the cheaper version just has a label which says "registered post" and zero other details, whereas (from definite memory) the more expensive postage option has a label which gives every detail, including the coin's value. That's the giveaway, and it's very obviously that which customs seize upon to impose a VAT charge, not only making your overall package much more expensive, but also causing additional delay while they faff around with it.
With the cheaper postage there is no clue as to what is in the packet - so unless they tear it open on a random check, they aren't going to know. They wouldn't have known in this specific case anyway, as there was no invoice, just the coin. So they would have been clueless about its true worth.