I'm guessing you are planning to have two rooms at the same time for the six days, and therefore London is as good a choice as any. I'm sure you will find enough in London for all to do, even for six days. There are also plenty of places you can get to outside of london without too much trouble. You could even take the Eurotunnel to Paris for a day trip!
Not sure about your choice of hotel, however I'm guessing that you don't have enough points for the GLONP. Kensington would be my choice out of the three listed.