Hi,
Your understanding of the overall position is correct, as confirmed in the following guidance:
VCM20020 - EIS: disposal relief: CGT exemption
The guidance states "The investor may not be able to obtain Income Tax relief because their total income is too low or their income tax liability is reduced to nil by other reliefs. If the investor does not obtain any Income Tax relief on a subscription for shares in an EIS company there is no CGT exemption for those shares. CGT exemption is available if some Income Tax relief is given even though the effect of the claim is to reduce the investor’s Income Tax liability to nil"
The following guidance and associated links provide further information:
VCM20040 - EIS: disposal relief: Income Tax relief restricted
You can also see more information here:
VCM14120 - Venture Capital Schemes: the Enterprise Investment Scheme: income tax relief: company procedures: attribution of EIS relief to shares
which states "Normally the amount of Income Tax relief attributable to a holding of shares will be self-evident. Where, however, the maximum amount on which relief is available for a year (see VCM10530) is exceeded, the position is not clear. In this situation the total relief obtained for the year is apportioned between all the holdings of shares for which relief was claimed, or the individual can choose to claim relief on some shares and not on others"
Thank you.