We did our POAs, initially, through a solicitor before we clocked that this wasn't necessary and could be done by us and we received a certified (solicitor stamped) copy which we had to pay for. If you are doing it yourself the copy that comes back from the OPG is certified and has a weird edging to it. The latter is a valid POA and can be shown to banks etc.
Per my previous post, nobody accepted the online access code, only the DWP and Attendance Allowance Office, so you'll need the hard copy.
Some organisations, such as broadband supplier and a utilities company, accepted an emailed pdf of the certified POA.
The 'jointly and severally' is the best to get as it means you can act on behalf of the donors (your parents) independently of your DB. Helpful if there is a distance between the attorneys. If you hadn't got jointly and severally you would need your brother's signature on everything.
Only the attorney should need to be in attendance to register the POA with a bank. I went to the bank to register my DF's POA and they didn't need him to go there but I had to show the hard copy POA and bring two forms of ID. The bank, I think if I remember correctly, took the POA, scanned it and then gave it back to me.
I recall, a couple of times, I was asked to send the certified POA hard copy to organisations but I refused explaining that it had cost £40 and I didn't want to risk it getting lost in the post and I was able to email a scanned copy. In a couple of instances where organisations have been difficult I have found paperwork and set up an online account and managed things that way, avoiding the POA hassle, which it sometimes can be.