You can't. They state somewhere in their Big Bad Help System that they only accept US dollar denominated cheques. Perhaps you can purchase money orders in US dollars? (I can't imagine they wouldn't accept those, but perhaps you'd better check). Are U.S. dollar denominated chequing accounts available in the UK? I know you can get one here in Canada.
Another (somewhat risky) option would be to send them US dollar cash.
Another option is you can Paypal me $8.95/domainyear and I will purchase the domain for you and push it into your eNom account. Send me a PM or email if you are interested.