We did a 5 star secret hotel last year in Hide Park area and no compaints whatsoever.
I would question the savings though as we booked a secret 4 star for next Friday night near St Pauls for EUR 104 and when I checked the hotels own website, same room was GBP 90.
I wouldn't worry about London being booked up. You could always stay in the City area if desperate as this always seems to be quiet on weekends. I'd recommend trying a lastminute tomorrow or even early Friday.