The calculation is carried out in 3 stages:
1. Customs duty is calculated as a percentage of the value of the goods. The rate is 2.5% for silver, gold or platinum jewellery. For 'imitation' jewellery it's 4%. So the (theoretical) duty is somewhere between 2.5% of £25 (63p) and 4% of £60 (£2.40). However the duty is waived if it comes to less than £9. So you've nothing to pay from that stage.
2. Next, VAT is calculated at 17.5%. It's charged on the value of the goods and, where appropriate on the duty. (VAT can theoretically be charged on carriage costs but it seems to be ignored for personal gifts). That makes the VAT come to somewhere between 17.5% of £25 (£4.38) and 17.5% of £60 (£10.50). VAT is only waived where the value of the goods is less than £18.
3. Lastly a customs inspection fee is added on. For low-value postal imports this is £8. (If there's no customs duty or VAT to be paid, the inspection fee is waived).
So the postman will demand between £12.38 and £18.50 before handing over your package.
If your friend wants to send you a gift, tell her to ensure that the value is less than £18. Then you won't have to pay a penny to receive it, as long as she has attached a correctly completed CN22 customs form to the package. (CN22 forms are available, free of charge, from post offices throughout the world). If it's possible for her to split up the jewellery, which she was planning to send, into several low-value packages (of less than £18 each) you'll be able to avoid any charges.
Chris