Ok, it's at least £26; that's an awful lot of money. It will however, allow you to be taken around London showing the main attractions. What o would do though, is to buy a book, I like the DK Eyewitness guide books, and identify what you need/want to see, what time you have in order to see them and plan your route using the underground or by walking. You can get a zone 1-4 travel card for 1 day for £12 and a week long one for £46 (these costs are approx but near enough). Oh, and I think I covers the bus too.
We used the hop on hop off bus in Rome. While it allowed us to travel around the city and decide what we'd like to see, wouldn't do it again. You may find as we did, we had to wait forever to hop back on the bus because you can only hop back in if there's space for you on the bus. about 3 years ago
My sister and I used this bus when we travelled to London in 2011. It is a bit expensive compared to the underground but you don't see much underground! We found it both convenient and friendly. Our hotel was able to get us cheaper tickets, not sure if that still happens though. about 3 years ago