Can I deduct donations to a British Charity such as their Blue Cross or Lupus UK? I have receipts. Donations were made through Just Giving. Thanks

No, I could not find Just Giving on the IRS website Exempt Organizations. See the link to search the IRS Organizations eligible to receive tax-deductible charitable contributions:  https://apps.irs.gov/app/eos/mainSearch.do?mainSearchChoice=pub78&dispatchMethod=selectSearch 

Related information: 

• Exempt Organizations Select Check https://www.irs.gov/charities-non-profits/exempt-organizations-select-check
  
• What is a qualified charitable organization?