It would be hard to beat St. Ermin’s for character and history. It is definitely NOT your standard Marriott. It is part of the Autograph Collection, meaning it has its own identity, and history that goes back way before it affiliated with Marriott. And, like other Autograph Collection hotels, it is not owned by Marriott, just affiliated with them for booking and points purposes. According to the Wikipedia article below, it is privately owned by a wealthy Asian family, or by their business.
Read about St. Ermin’s history, including its role as the place where Churchill started the Special Operations Executive spy unit in 1940 (later moved to Baker Street):
https://www.historichotels.org/hotels-resorts/st-ermins-hotel/history.php
https://en.wikipedia.org/wiki/St._Ermin%27s_Hotel
https://en.wikipedia.org/wiki/Special_Operations_Executive ( see the section under “Origins”.
We stayed here for 3 nights on one of our many visit to London, and loved it. The hotel is charming. Our room was on the 3d floor, and we passed the window onto the rooftop bee garden as we walked the hallway to and from our room.
https://www.sterminshotel.co.uk/about/bees-kitchen-garden
The location, between Westminster Abbey and St. James Park, is ideal for walkers, and also convenient to 2 tube stops, Westminster and St. James. There is also a stop nearby for several useful bus routes, the 11, 24, and 148. We prefer the bus to the tube; it may be slower but at least it is above ground, and you can sit upstairs and enjoy the view.
https://content.tfl.gov.uk/bus-route-maps/key-bus-routes-in-central-london.pdf
It is also convenient for riding the Uber Thames boats as Westminster Pier is nearby.
The one drawback is there are very few pubs and restaurants in the immediate vicinity, other than the in-house Caxton Bar and Grill ( apparently Churchill enjoyed a drink or two at this bar, so more history there.). Fuller’s Pub, in the Sanctuary House Hotel on Tothill St. is an option for a pub. But mainly you will be going elsewhere for your dinner. There are lots of options in the “casual”:category just across the river—-take the Golden Jubilee pedestrian bridge, attached to the Hungerford Railway bridge, and turn left when you reach the other side. Or cross through St. James Park and wander up towards Soho and the West End area.