If you don't use a credit card, you need to have the amount converted to pounds sterling. This is from Chiltern's site:

3. Payment methods

You may order online using a credit / debit card (we accept MasterCard/Eurocard, Visa, American Express, Switch and Delta). You may order by fax, telephone or post using credit / debit card, cheque or postal order. All prices are in pounds sterling (£) and credit / debit card transactions for orders outside the UK will be converted by the card company at their current exchange rates. Cheques and postal orders should always be in pounds sterling made payable to Chiltern Seeds.