Hi Sixto Olson,
If you are resident in the UK and have any foreign income, regardless of how much there is, it must be reported in a self assessment tax return.
The disposal of the share, may result in a capital gains tax liability, which should be reported on a self assessment tax return.
You will need to convert the buying and disposal value, plus buying and selling costs to pounds sterling, using the official exchange rate in place at the time of buying and selling.
The official exchange rates can be found at Exchange rates from HMRC in CSV and XML format
and archived rates at Exchange rates