Source: Paul Bence, Flickr. Licence.
All books, dissertations and other physical resources in the SIA London should be found by Summon, but there may be occasional errors. In addition, new items may take a few days to appear in Summon. It's possible to search for physical resources in the book catalogue, which is complete and up-to-date.
Summon sometimes has fuller information about the contents of a physical book than the book catalogue; and sometimes it is the other way round. So when researching a topic, it is sometimes advisable to search in both places.
For dissertations from other colleges and universities, Summon will find many, because it includes ProQuest Dissertations and Theses Global: Humanties and Social Sciences and also EThOS, which gives access to UK doctoral theses. However there are many other options to try, listed in the other dissertations and theses guide. In particular, Global ETD is a good place to start.