Thanks, ML ;-)
To be sure of avoid paying charges you need to keep the value of the goods to less than £15.
Here's how the charges are worked out:
Firstly, Customs Duty is calculated. The rates vary widely, depending on the nature of the goods. (There are over 100,000 different classifications!). They can also depend upon which country the items have been sent from. For example, cotton-based clothing sent from most countries (including China) attracts Customs Duty at 12%. However if the clothing had been sent from the USA the rate would be 27% because the USA breaches the rules of the World Trade Organisation by subsidising its cotton farmers. (I'm unaware of any similar penalties applying to China but, if they also break WTO rules, there may be some goods which also attract such higher charges).
However Customs Duty is waived if the total duty comes to less than £9. (If the items are gifts it's also waived automatically if the value of the gifts comes to less than £135).
Next, VAT is calculated. It's charged on the same goods, and at the same rates, as would be charged for purchases within the UK. So there's no VAT on books and children's clothes but most are items are subjected to VAT at 20%. It's also worth noting that VAT is charged on the carriage costs to get the items to the UK and on any Customs Duty to be paid. (Yes, you're right! That really is a tax on a tax!!!).
However VAT isn't charged where the value of the goods is less than £15 (or £36 for bona fide gifts).
Lastly, an £8 customs examination fee is added on. However that's waived if there's no Customs Duty or VAT to pay.
So it works out in practice that £15 is the cut-off figure for the value of non-Eu imports if you want to avoid charges. Something worth £16, that cost £4 to send here, would avoid Customs Duty (because of its low value) but would be charged £4 VAT and an £8 examination fee, totalling £12 in all.