Having foreign income, regardless of how much it is, is criteria for completing self assessment tax returns.
Untaxed foreign interest of less than £2000.00 can be entered in box 3 of page TR3 in SA100. Foreign dividends of up to £2000.00 can be put in box 6 on page TR3 of SA100. If tax was taken off of the dividends, it can be entered in box box 7.
The disposal of share can give rise to capital gains. The annual exempt allowance (AEA) for capital gains is £12300 in 2022/23 and £6000 in 2023/24.
If your disposals in the whole tax year mean that your gain is below the AEA, you do not need to include in your tax return.
Guidance on completing SA100, can be found at How to fill in your tax return (2022)