Hi Andrew Nelson,
The company may have issued a circular or prospectus to its shareholders (both resident and non resident).
This should include the company’s explanation of the UK tax treatment.
Generally, for capital gains purposes a share reorganisation is not treated as a disposal of the taxpayer’s existing shares or an acquisition of any new shares and new shares issued are treated as though they were acquired at the same time as the existing shares.
You may need to apportion the allowable cost from the respective values of the shares, see:
HS285 Share reorganisations, company takeovers and Capital Gains Tax (2022)
There is more on foreign income here:
SAIM1130 - Savings and investment income: foreign income Territorial scope