None.
Many people get caught out with unexpected charges when they import items from non-EU countries, but you won't be one of them.
Theoretically, you pay 12% duty on the fabric, plus a further 15% 'ad valorem' duty on cotton materials because the USA is currently breaching World Trade Organisation rules on subsidies to its cotton farmers. (i.e. the UK has imposed a trading sanction on cotton imports from the USA). However, Import Duty is waived if it comes to less than �7.
Then, theoretically, you pay VAT (at 17.5%) on the cost of the goods, on the cost of the carriage and on the duty. (Yes, that means you pay tax on a tax!). However, VAT is waived if the value of the goods is less than �18.
Lastly, you pay a flat-rate fee, of �8, for the examination of your package. However, the customs examination fee is waived if there's no duty or VAT to pay.
So, you won't have to pay anything.
(I'm sorry that this has been rather long-winded. The correct answer was wholly within the first word, but I thought it best to warn you about the charges that you could face if you later ordered more expensive goods from a non-EU country).
Chris