You have a couple options for figuring out the London portion of your trip. First off, I would recommend picking up a copy of Rick's book: Europe Through the Back Door.. This book is focused on travel skills and tips (such as how to use the subways, money tips, safety tips, etc.). It also includes sections of many of the highlights of the major tourist locations in Europe (including sights, hotels and restaurants, etc). This would be a good read for you. I'm sure your library will have a copy.
Second, you can think of your stay in London as a "mini-vacation" before the tour starts. This means, you read guidebooks, check websites like this one and Trip Advisor, etc. and then you figure out hotels, restaurants, and sights in London you want to see. You can email/call the hotels directly to make your own reservations and plane rickets. Then you arrive at the airport and you're off! This would be the same as if you were planning a trip to NYC or Disney or Washington, D.C. (Or wherever in the US -I'm not sure where you're from and would go on a trip). This sounds intimidating, but you can do it if you want. Asking questions here and on other travel sites as well as reading guidebooks will help you pick safe neighborhoods, good hotels, etc. Obviously, costs will depend upon what you want to see - Tower of London, London Eye, etc.
If this sounds too intimidating you still have another option. RS offers "Tour Consultations" where they recommend hotels, etc but then you have to call and make the reservations with the hotels themselves. It costs money, but the nice thing about this is, if you go on one of their tours, it's 50% off if you arranging pre- and/or post- tour stays. Here's the link:
I haven't taken the London to Florence tour, but from Florence, you can again, plan on your own. You can just hop the train and head to each city (with some planning, etc). Several people on the Helpline/Forum are more experienced with the train schedules, so you can post those questions, under the "Transportation" section of the Forum. Same goes with questions regarding: hotels, neighborhoods, sights, etc.). Just click on 'Travel Forum' on the far left of your screen and then chose the appropriate board for each question.
Unfortunately I have not been on an Insight tour, so I can't help you with them.
Again, I realize this sounds rather intimidating if you are not an experienced traveler, but if the planning does sound like it might interest you, I would definitely recommend going for it! You can get lots of help here!
You can also watch the
Travel skills videos (3 parts) on this website for a basic introduction to using the trains, etc.
https://www.ricksteves.com/watch-read-listen/video/tv-show. (On this page, scan to almost the bottom of the list of videos and choose 'Travel Skills'). It will open up to all three
I forgot to mention, if you decide to take a tour when you buy your plane tickets, you want to search for tickets using the "Multi-City' option. This will allow for you to buy a ticket from your hometown airport into London and out of Venice (or Florence or Rome). Often this is cheaper than a roundtrip into/out of one city.