Disclaiming inheritance under US trust / will & UK IHT

My client is due to inherit $500K under her late sister’s US trust. She wishes to disclaim half so that under the US ‘waterfall’ rules, that half would go to her children.

Under UK IHT rules, a disclaimer cannot be of part of a gift in a UK will, but must be for the whole amount. Would a partial disclaimer of a US inheritance result in HMRC deeming the money as being in her estate for IHT purposes and, if so, treated as a PET?

Thanks