Q. How do I find a thesis by topic or by research method?

You can search for IOE theses in UCL Explore

For more detailed searching you can use Digital Dissertations and Theses database to search by keyword in the abstracts as well as the title. This will help you to find theses by topic or by research method. 

You can also limit your search by 'school' in Digital Disserations and Theses by typing Institute of Education University of London in to the School search box.  Once you have identified the theses you are interested in you can request these through UCL Explore as usual.