I agree with Andrew Goodman. There are a host of reporting documents for the US resident (I assume a child of the deceased, and who also is a US permanent resident (Green Card holder) and who still holds a British passport). This in turn means FATCA can come into play with additional reporting requirements not only if the US resident is a trustee, but also if the US resident is just a named beneficiary of the trust. Forms W-8BEN-E; 114; 8938; 8621; 5471; 8858 and 3520 & 3520-A spring to mind to be considered.
The UK Trust itself as it will have a US resident beneficiary, will also have to file Form W-8BEN-E at the very least.
If the trust in due time holds UK shares you may find that because of FATCA and the presence of a US beneficiary / trustee that some banks (and certainly some stockbrokers) will refuse to handle the Trust’s affairs because of the onerous FATCA reporting requirements.
This area of US tax law is a minefield and an expert is necessary (but even the experts are not always right, these days).
Peter Double / Probate Resealing Services.