MissAnonymous

Hello, I'm an individual investor looking to buy and sell US stocks as a UK citizen in the UK. I have a W-8 BEN form to half my withholding tax. My question is will I have to pay both US and UK capital gains on any gains made, or is there a tax treaty that means I only have to pay capital gains in one country? And if so, which? Is withholding tax automatically deducted or would I have to pay that through self-assessment with the IRS? Incidentally, if I own US stock long enough that it earns dividends, do I have to file for income tax in both countries? Thank you for your time.