I just returned from this tour, and I think you may have your days mixed up. The day you take the river boat cruise is the first day you are in London (after arriving by train from York). It begins with train ride from York to London, then Tower of London, then boat ride, then the bus holding everyone's luggage picks you up and takes you to your hotel.
If you don't want to go on the tube orientation walk, you could break away as soon as you get your room key and drop your bags in your hotel room. For us, we reached our hotel that day about 2:30-3:00.
This is NOT your last full day of the tour.
The following day (visit to Westminster Abbey) is your last full day of the tour. It is labeled Day 13 of the tour. You will go to the Abbey first thing that day, then when you are finished touring it, you are released to be on your own, for us this was 12 Noon.
Go to this page, then click on "Itinerary":
https://www.ricksteves.com/tours/england-scotland/england
The following is the schedule given here on the RS website for the last two full days of your tour:
From the RS website:
Day 12: Tower of London and the River Thames
Today we'll board a train and make a beeline past the Midlands for London. Upon arrival, we'll jump into the city's medieval past, with a visit to the Tower of London, where you can join a tour with a Beefeater and get a close-up look at the UK's collection of crown jewels. Then we'll take an entertaining cruise on the River Thames, hearing tales about the London Bridge, St. Paul's Cathedral, and Big Ben. Later this afternoon, we'll check into our hotel and get oriented to our neighborhood and London's Tube. Tonight is a perfect evening for the theater (book something in advance online or pick up a ticket in London). Sleep in central London (2 nights). Train: 2 hours. Boat: ½ hour. Bus: 2 hours.
Day 13: Westminster Abbey
This morning we'll meet with a local guide to visit Westminster Abbey, where England's kings and queens have been crowned and buried since 1066. A thousand years of English history lie within its stained-glass splendor and under its stone slabs. This afternoon is yours to explore the Churchill War Rooms, stroll past Buckingham Palace, tour the British Museum, or take a walk through one of the city's wonderful parks. Tonight we'll meet for a final group dinner together, where we'll share travel memories and toast new friends. Cheers! No bus. Walking: moderate.
Note: If your schedule you were given for your tour has things re-arranged in a different order, then I don't know what to tell you.
We were not released when our river cruise ended at Westminster Pier.
We still had to go by bus to our hotel, be assigned rooms, and drop bags. Either of the last 2 days of the tour, you could squeeze in Buckingham Palace.