The showstopper is that they want to see either her Driving Licence (she had never driven), or her Passport (not sure when it expired but I suspect it was more than three months ago). The girl also mentioned an alternative - a Residency Permit. What exactly is that, and can Mum get on pretty sharpish?
And there's the kicker. When my Dad died and the bank account was only in his name, trying to get my Mum to take over the account was horrendous. As you've said, no driving licence, passport expired years ago, they wouldn't take the post office account, where she had her pension paid into, as proof.
Probate had given her all the assets but she couldn't touch them or draw money out. Thank goodness for the post office account.
Finally, the bank manager said if she knew someone with an account at the same bank who could sign to say they knew who she was they would accept that. Luckily enough, my sister in law had an account at the same branch and had to sign a statement saying she personally knew my Mum.
Everything else (utility bills all in my Dad's name) seemed to elicit the response "computer says no", even letters from the council weren't accepted as they weren't on the approved list.
Nightmare. Best of luck in getting it sorted.