Help with London Hotel Location

Kensington is my favorite area of London. It's an upscale neighborhood, but not too expensive compared to Soho or Mayfair. Many of the hotels are designed to blend with the architecture of the district, so when you're away from the high street it feels residential. In addition, it's central location makes it only a tube stop or two away from the major attractions. Follow your mom's instincts and stay at a B&B or traditional hotel. You'll get more out of the experience - providing you can find one without a lot of stairs.

Nadler has three 4-star properties in London, at Kensington, Victoria and Soho. Well reviewed and great value in a city where a bit of luxury normally costs an arm and a leg.
https://www.nadlerhotels.com

For accessible tube stations.
https://tfl.gov.uk/transport-accessibility/wheelchair-access-and-avoiding-stairs

I'll second the Nadler Hotels. I've only stayed at the Kensington one, where I'm a semi-regular, but the manager there recently moved over to their Soho branch.

Another Nadler is under development near Covent Garden/Strand area but I'm not sure when it will open.

Great service, charming properties, mini-kitchenettes in every room.

The Nadler looks like it could possibly work although the decor might be a little more modern than my mom might prefer. Which location would be the best? The Kensington location near Earls Court would be easy for getting to and from Heathrow and that station is accessible but it's not as central as Soho.

Earl's Court would be a good area for the reasons given above. London sights are so spread out that there's no idea "central" location to stay. You'll be using the tube and/or buses wherever your hotel is -- assuming that you want to visit the places most people go, like the Tower, British Museum, Buckingham Palace, Westminster, St. Paul's, etc. etc. Earl's Court is between Heathrow and most of those sights, but has three tube lines with varying degrees of accessibility. Once on the tube you can get anywhere fast -- again subject to the need to minimize stairs. There are also several bus lines.

Taxis are the easiest way to travel without stairs or excessive walking, but they're expensive and can be slow if traffic is bad.

You mentioned in your original question,
"My mom will be 77 at the time of our trip. She is very active but would prefer to avoid too many steps."
I wanted to warn you that the Tower of London has a great many stairs. Stairs to get from one floor to the next inside the White Tower. Stairs to get up to walk around the outer walls and stairs to get up to see the living quarters along the front wall. Quite steep spiral staircases inside some of the turrets along the wall, with no handrail.

My recommendation for a hotel is the Principal, a hotel that was built in 1898 and until recently was called the Hotel Russell. It faces Russell Square, a green leafy park, in the neighborhood of Bloomsbury. It is two blocks from the British Museum.
https://www.phcompany.com/principal/london-hotel/
The hotel was designed by the same architect who designed the interiors of the Titanic. Its restaurant is said to be almost identical to the RMS Titanic's dining room.
This is an old classic historic hotel that has been recently renovated.

The Russell Square tube station is on a direct line from Heathrow.
This location is a quiet residential neighborhood, with close access to restaurants and transportation.

I recommend a room facing the front for a view of the park, on the 4th or 5th floor to reduce any street noise (there shouldn't be much in the middle of the night.).

Green Park station on the Piccadilly Line now has easy access for possibilities in Mayfair such as the Chesterfield.
https://www.chesterfieldmayfair.com/

There are very few Underground stations with step-free access still, so even if you stay next to one with access you probably still won't be able to go anywhere much by Tube due to lack of access at the other end of your trip.

We can recommend The Thistle, Trafalger Square. Immediate next to National / Portrait Galleries, Traflager Square, Leicester Square, Chinatown, easy walking to Horseguards, Whitehall/Chuchill War Rooms, great tube stops. We had excellent service and a very good room. Check out location on a map.

Kathleen,

My favorite area to stay in London is between Earl's Court and Gloucester Road tube stations.

There is a Holiday Inn hotel almost exactly halfway between these two tube stations. While the Gloucester Road station did not have a lift the last time we were there, the Earl's Court station did. This was very important as my 80yo mother had difficulty with stairs.

The Holiday Inn (Kensington Forum) wasn't particularly charming, it did tick all the other boxes. And its location can't be beat. A very short walk to the tube stations and on a major bus route. In fact, after a long day of sightseeing, it was nice to walk out the back of the Earl's Court station and board a bus that dropped off directly in front of the hotel.

One of the most conveniently located hotels in London is the Hilton London Paddington Hotel, connected to Paddington train station.
It is a Victorian historic hotel with elevators.
http://www3.hilton.com/en/hotels/united-kingdom/hilton-london-paddington-LONPDHI/index.html
Paddington is the station that the Heathrow Express train comes into from Heathrow Airport. Likewise, it is the station from which you may leave and go to Heathrow Airport on your day of departure.
Paddington Station area is served by tube and bus lines, as well as having taxis waiting.
I have stayed at this hotel; I liked the location and the clean, soft beds.