My mother had a huge number of accounts, and I think every one was different, and information provided by a telephone adviser was inevitability wrong. I don't think I got any sorted within a single visit.
Take in as much documentation as you can. Your passport, a recent utility bill, and a driving license. The ORIGIONAL POA (let them photocopy it), and your mother's identification documents (passport etc). Some banks stipulate two forms of photo ID.
Not all branches do POAs. Sometimes you have to make an appointment. So worth checking in advance.
You fill in their form, and they send this and the POA to Head Office. It can then take a little while till you get written confirmation that your name has been added. The forms can be complicated, so I fund it useful to do in branch.
I understand that some banks can be funny if the POA responsibility is shared. I think it then becomes complicated for them. It may be worth checking in advance.
I got really stuck with one bank. The computer systems for the investment product my mum had, did not include scope to register a POA. But I could not close it without the POA being registered. A nightmare. My advice is to do them all as early as possible. And perhaps try for bigger City Centre branches.
(An alternative, which I did before the POA came through, was to take my mum plus passport and driving license into the bank, sit her down, and then go up to the cashier, and get my mum to sign whatever was needed to get the money transferred somewhere more accessible.)