Hi! My twin sister and I are also Boomers, and huge Harry Potter fans. We just returned from the Harry Potter Celebration Event at Universal Orlando for our second time. We are departing the US and heading to England just two days from now, on March 1, 2015. Naturally, the WB Studio Tour will be a big highlight of this trip. I did not see what dates you are planning to travel, but hopefully it will be AFTER March 19th when the new Exhibit is revealed of the original Hogwarts Express at WB Studio! My husband and I spent exactly eight hours on our first visit to WB Studio in Leavesden in 2012. The thing is, you cannot really compare Universal to WB Studio, since the first is an enormous amusement park, and WB is simply two enormous sound-stages plus a back-lot, crammed full of every incredible detail from the Harry Potter movie sets. There is no HP experience in the world like stepping into The Great Hall.
So, my recommendation is to buy the first time-slot available on the WB Studio website for the day you have completely open on your trip. We had a rental car and found it very easily without having to rely on anyone's schedule but our own. Which means I cannot help you with connections. But I would like to mention that many, many HP movie sites can be found in London simply by walking around on your own with a good guide book, including Rick's! We saw the entrance to the Ministry of Magic, the Australia House which was Gringotts Bank, Cecil Court which is an original Diagon Alley, the Millennium Bridge (watching for Death Eaters to blow us up), and the original Leaky Cauldron front door in Leadenhall Market, as well as the Leaky Cauldron from Azkaban at Borough Market. We also crossed the Lambeth Bridge where the Knight Bus squeezed through traffic during its manic race to the Leaky Cauldron.
I agree with posters above who warned against the Golden Tours Bus. You can do this all on your own! First step, log onto the WB Studios Tour website and buy those tickets! HAVE A FABULOUS TIME!
PS The email address of mine is at work, and I will NOT be there for the next 3 weeks. England here we come!