In a General Account, any dividends and/or interest that your investments generate are taxable in the year you receive them, and you'll have to pay the tax with your next self-assessment.
Basic-rate taxpayers are entitled to a £1,000 personal savings allowance each tax year, higher-rate taxpayers are entitled to a £500 allowance and additional-rate taxpayers do not have any savings allowance. Interest income above this rate is taxed at standard income rates.
For dividends, taxpayers are entitled to a £2,000 dividend allowance each year. Any dividends above this allowance are taxed at 7.5% for basic rate taxpayers, 32.5% for higher rate taxpayers and 38.1% for additional rate taxpayers.
When you eventually sell your investments, you will need to pay capital gains tax if your gains exceed the HMRC limit, which is currently £12,300 for the 2020/21 tax year.
Capital gains tax rates are 10% for basic-rate taxpayers and 20% for higher-rate taxpayers.