To date, we don't have a formal answer on UK taxes for Americans not living in the UK.
We believe that non residents of the UK (like Americans living in America) are not subject to local UK taxes on their UK forex trading gains. There might be UK tax withholding on interest only - and dividends if on stocks too - but many forex traders in the UK don't earn interest on their forex accounts in the UK.
It's a similar tax rule in the US for UK or other non resident aliens of the US, with US-based trading accounts. Non resident aliens don't pay US taxes on US trading capital gains. See
TRADER TYPES: NON-RESIDENT.
Generally, countries don't tax capital gains activities by non resident aliens unless the non resident alien spends considerable time in the country - it's 183 days in the US.
Americans do need to file a US foreign bank account FBA form reporting their UK and other foreign accounts - including the FXCM UK platform account in our view. TDF 90.22-1. There are very large penalties if you omit filing this form when required. Learn more at
Report of Foreign Bank and Financial Accounts (FBAR).