Recognizing, up front, that there may be massive differences between law in the U.K. and here in the U.S., I would say that often, it's only major holdings that are important when helping a person with their affairs.
By that I mean, Power of Attorney isn't needed for paying their bills out of a checking account or transferring money from a savings account to a checking account, as long as the person performing those duties is listed as a co-owner of the accounts. Additionally, if there's real estate involved, especially if it's "free and clear" from hindrances, such a mortgage, the person, again, performing the duties can be listed on the title of the property as a co-owner. Same with automobiles and the like.
When my own parents were becoming elderly, they asked me to hold power of attorney. After talking to their own attorney, it was decided just to enter my name on each of the accounts/titles rather than go through the problems encountered with power of attorney, such as getting a bank or title company to recognize your "power of attorney" appointment.
Once one's name is legally entered on the various accounts, there's no longer any argument. Additionally, when my parents died, final payment of their final bills as well as disposing of the properties became a relatively simple matter... much easier than holding power of attorney when both parents had died and could no longer verify the appointment.
Several factors made this the best choice for us... one major one being that I was an only child and didn't have to deal with dissaffected relatives. It's amazing how a previously loving and caring family can be torn apart when money and/or property is involved. Again, all of that is no longer an issue, since the co-ownership cannot be contested whereas power of attorney can... and sometimes rather easily.
Best of luck!