Dear MrMichael – very sad to hear this news but wishing you all the very best with your battle. I have always enjoyed reading your posts very much, and tend to agree with much that you post. My best wishes to you and your family.
The Chelsea Harbour Hotel may possibly work for you. Now managed by Millennium Hotels.
They have River View Suites, that they advertise from £290, though suspect they may cost more. They measure 48 sqm / 517 sq ft, though you may need to request a dining room table in place of the sofa / easy chairs in the living area of the suite.
They have a balcony and should offer a reasonable view of the marina and River Thames.