Re your proof of identity, I got my passport for this very reason, I haven't been abroad for years but it is useful ID. When dealing with LPA issues I was sometimes asked for passport/driving licence plus a utility bill.
If you want certified copies of the LPA, a solicitor will provide them for you (they make a charge, my mother's solicitor charged £25 per copy). I had several certified copies made, so I could send them in the post as necessary (I had to send out quite a few copies, e.g. to my mother's landlords, the DWP re Attendance Allowance, various financial institutions). They are supposed to return them to you but I only had about 1 in 2 returned. So definitely don't let the original out of your sight.