Capital gains on US stocks from UK (double taxation?)

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.

Hi,
You may be liable to pay tax in both countries but if you do, you can then claim Foreign Tax Credit Relief for the tax paid abroad to reduce your UK liability. You can see guidance here:
Tax on foreign income
Thank you.

Not HMRC...search US DTA...article 13 as to which gains are liable where,,,where a paragraph says "only" its that country only and doesnt go on the return in the other country and no tax credit due as its not taxable in the other, where a paragraph says the country "may" tax it it means that country/both can/will/is liable in both with tax credit in other.