Import Duty on most types of clothing, from outside of the EU, is (as you state) usually 12%. However, the USA is currently in breach of a World Trade Organisation ruling relating to subsidies to cotton farmers. The UK is therefore applying sanctions against the USA, in regard to imports of cotton-based products. That means that the current rate of import duty is 27%. (Duty is waived if it comes to less than �7).
VAT is charged, at 17.5% on the value of the goods, on the carriage charges and on the duty. (Yes, that's a tax on a tax!). VAT is waived if the value of the goods is less than �18.
If there's no duty or VAT to pay, you don't have to pay a Customs examination fee. Otherwise you pay �8.
Let's suppose you buy cotton based clothing for $200 (�100) and you pay $40 (�20) for the carriage. Here's how the sums work out:
Duty = 27% of �100 = �27
VAT = 17.5% of (�100 + �20 + �27) = �25.73
Examination fee = �8
Total to pay the postman = �27 + �25.73 + �8 = �60.73.
Chris