Hello, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
For whom did you complete his form regarding tax at 15%? You should repudiate it immediately for as far as the UK is concerned it is simply balderdash and piffle. Under the UK Capital Gains Tax (CGT) regime in any one tax year the first 11.1K of gains are tax free and anything over, except residential property gains, is taxed at 10% or 20% or a combination of the two rates depending on your income including the gain in the relevant tax year. You declare your capital gains on your self assessment tax return which as a self employed person you will have to complete after 5 April annually.
You are not liable for any UK taxation on moneys withdrawn from your US brokerage account to your UK bank account. If you earn interest on your UK account that is liable to Income Tax (IT), Here is the Gov UK advice on the subject:
'Personal Savings Allowance
If your total taxable income is £17,000 or less you won’t pay any tax on your savings income.
Otherwise your allowance depends on which Income Tax band you’re in.
Income Tax band Tax-free savings income
You’ll pay tax on any income above this at your usual rate of Income Tax.'
Deposit takers have not deducted automatically since 6 April 2016.
I do hope that I have cleared the air for you on this matter.