i'm very sorry to hear about your mother, my condolences...
So very difficult when you're thousands of miles away, totally overwhelming.
The (i'm guessing) 401(k) custodian can send you a check in the U.K. It's totally unreasonable and virtually not possible to expect a non U.S resident beneficiary to open a U.S. bank account. You may have been able to convert your mother's account to yours, perhaps you are ITF/POD on the account. This would have been the simple option for other financial items in the estate too. Once the estate is out of probable and closed, so can the bank account and a check mailed to you. You can of course open a Wise account as suggested and ACH to yourself. Also
https://www.sdfcu.org/ will open an account for you, should you prefer a long term, traditional approach.
The 401(k) will be a part of an inheritance.. Just call HMRC and ask how it's handled, they've heard it all before...
I hope this helps and best wishes....