Hi Claire. if you wish to claim any VAT you have to be registered with VAT HMRC office.
if you provide only services to America – your sales are outside the scope of UK VAT, so there is no obligation to register for VAT in UK. However, you can register for VAT voluntarily. Once registered then you can claim VAT charged on you but you have to weigh the benefits of registering voluntarily with the regulatory obligations of filing the returns every quarter.