If you buy shares, you will need to complete a tax return. If you have any doubts or questions over this please take independent financial advice or contact HMRC.
You may have to pay Capital Gains Tax (CGT) if you make a profit when you sell shares. Dabbl will provide a statement to you showing whether you made a "gain" for the tax year. Our fees and any Stamp Duty Reserve Tax incurred will be deducted from profits realised. If your total gains are above your Capital Gains Tax allowance for the tax year, you may well have to pay tax.
For more details see https://www.gov.uk/tax-sell-shares/what-you-pay-it-on