The important one is the Financial. I don't think anyone has ever asked to see the Care one. However if you are doing one you might as well do both.
Technically you don't need a lawyer. Information is here:
www.justice.gov.uk/about/opg
Indeed it looks as if you can do it on line. If things are pretty straightforward I would not bother. There will be lots of advice on places like:
forum.alzheimers.org.uk/forumdisplay.php?60-Legal-and-financial-issues or Age UK.
My issue is that there will be an inheritance so I am in effect managing money for other people, not just my mum. We did not use a lawyer, as things needed to be done quickly following my mother's fall. I wish I had insisted we did, as it would have been useful for me to understand better the responsibilities I was taking on, and the boundaries of what might be considered reasonable decision making, eg should I be using Financial Advisers or is it OK to simply do what I would do if it were my money. And what is my liability should I take money out of property, because I am sick of managing property, and put it into a "safe" stock market tracker and then the stock market crashes. (Or vice versa. Attorneys apparently have a duty to manage assets actively. The silver lining is that I have now read up on financial stuff sufficiently that we are a lot more savvy about our own pensions, mortgages etc.) If an Attorney needs legal advice they have to pay for it themselves and cannot, apparently, reclaim as an expense.
Actually DH has been amazingly tolerant of DM. But it does no harm for her to think she has to be careful. The art of the white lie. Explore a little and see if there is something that works. Deep down the fear of abandonment must be pretty universal and so, if the right constraint can be found, it can form a useful part of any negotiation. You need to find a solution that works for you. There is little point in giving way to her demands only to find yourself in an impossible situation.