UPS will charge roughly the same as your postman would.
If you purchase goods from a non-EU country, the staff who process your import will start by calculating import duty. That varies depending upon the nature of the item being imported but it can be quite high. In particular, while cotton-based clothing from most countries is taxed at 12%, the USA is currently breaching World Trade Organisation rules by providing government subsidies to US cotton farmers. So, in order to counter any unfair advantage that US producers of cotton goods might have, the UK currently levies 27% tax on cotton clothing from the USA.
Then VAT is added to the INCLUSIVE cost of the purchase. (i.e. VAT is charged on the import tax, and on carriage charges, as well as on the cost of the goods).
Lastly, you're charged a fee for processing the import. The Post Office fee is £8. I believe that the UPS fee is similar.
So, if you were to buy a cotton-based garment from the USA for the equivalent of £100, with £10 carriage, you'd have the following to pay to UPS (or to your postman):
Import tax = 27% of £100 = £27
VAT = 17.5% of (£100+£10+£27) = £23.98
Processing fee = £8
Total to pay = £58.98
I've simplified things for the above example because there are exemptions from tax and VAT for low value items (and, if there's no tax or VAT to pay, the processing fee is also waived) but it illustrates the way that the fees are calculated.
Chris