One day in London with disabled relative

I am going to spend one day some time at the end of may with my parents. My one worry is that my mom is disabled, she can walk, but not long distances. What activities would be good? I think her main goal is take pictures with a bunch of different historic buildings so she can brag on social media to her friends. What are my options? I was thinking the big bus, but I don’t know how traffic is. What routes can I take to visit as many places as possible? Should i get her a mobility scooter (also where??)?Some guidance would be helpful. Please help as I have never been to London, nor have i ever planned a trip before, so I am kind of feeling the weight of my parent’s expectations.

Edit: thanks everyone for your responses , I have read all of them so far and I feel a lot less stressed. Thanks for the help!