Library Hub Discover will find you books in other libraries in the UK (plus Dublin).
Library Hub Discover does not tell you which of the libraries it lists you can access. For this info see:
The key libraries in London are likely to be:
MA students can access the online resource The Grand Tour at Senate House Library.
If you are researching the collecting policy of a museum or gallery, that institution's own library and archive will be important. All will have some kind of archive, even if no information about it appears on the institutions's website: do enquire.
WorldCat will find you books in other libraries worldwide. Coverage is uneven; e.g. there are some major African cities with no libraries included.
The SIA librarians will help you find books and access libraries anywhere in the world: though please note our knowledge of many countries is limited.