Library Hub Discover (LHD) enables searching of the collections of major UK academic libraries, all at once. You can read info about LHD here, including a list of some London libraries not included in LHD. Not every library listed gives free access to the public. However there is at least some free access to everywhere listed in the libraries in London guide. The website of each library will give further information.
If you live in the UK, it is also worth joining a public library. Scholarly material is available online, though only on the libraries' own computers, via the database Access to Research. Some other useful resources may be available from home once you have joined a library.
Worldwide, over 10,000 library collections can be searched via WorldCat. Some countries are better indexed by WorldCat than others. A small proportion of items are available to read online. Art Discovery searches WorldCat's art libraries only.