This is a great tour! You're going to have a wonderful time!
We took this tour a couple of years ago and the Radisson Blu, South Kensington was our last hotel of the tour.
Breakfast on the final morning was from 7 AM until 10 AM, a buffet. We were up early and ate about 7 to 7:30. Several of our tour mates were sharing the breakfast table with us, as well as our Rick Steves guide.
We were expected to return to our hotel room sometime after breakfast and get our bags, meet the guide at the front desk, and check out of the hotel. The guide was standing by to help the hotel staff sort through all the tour guests who were checking out. Some were staying on, and they needed to check out (end of RS paying for their room) and check back in on their own credit cards.
We were checking out and going to a new hotel in another neighborhood in London. We liked South Kensington, but wanted a quieter area for the extra 3 nights stay. Then after that, we were going to Windsor for 2 nights at a hotel there before flying home from Heathrow.
There are no group activities on the last morning of this tour. You will be free after breakfast to do as you wish.
Will you be moving to another hotel or staying a couple of extra nights at the Radisson?
South Kensington is a good location for visiting museums (V&A Museum, Natural History Museum, Science Museum, Clockmakers Museum) and you have the tube station across the road from the Radisson with several tube lines running through....easy to jump on and get to Heathrow airport for departure.
P.S. You do not have to attend the breakfast on the last day, but it's paid for in your tour so you may as well have a cup of coffee or tea and croissant before running out the door to see some sights. The museums don't open before 10 AM, so it's not like having breakfast will prevent you from getting in early to one of those.