Swiss dividend withholding tax on shares in an ISA

Swiss tax authorities will only refund 20% of the dividend tax paid using Form 86 for Swiss shares held in an ISA. If I wish to claim the remaining 15% of withheld tax from the HMRC how best is it to do this using my self assessment form. I want to be able to do this without the declared ISA dividend amount received not impacting my UK personal tax ie I do not want to end up 1) paying 8.75% (or more) UK dividend tax on shares held in an ISA OR 2) the ISA dividend amount unintentionally impacting my UK tax threshold by pushing me into the next tax threshold

Hi,
HMRC wont refund foreign tax deducted and can only give a tax credit for the tax paid if there is any UK tax due on the same source of income. As income within an ISA is tax free, there will be no refund due.
Thank you.