We went for the ladies day spa - great value for money and a very relaxing day. One point was that there were a number of swimming lessons for kids which made the pool area not as calm and was not expected from a spa.
The Blue Harbour Spa is one of the most relaxing, intimate and peaceful Spa's in London. The venue is well kept, clean and modern. The staff are expectationally friendly and go beyond just being polite. I am always recommending Blue Harbour as a Spa of choice for my friends.