To locate material not held by UCL you can use the Library Hub Discover site: an online catalogue with records of the holdings the top research libraries in the country.
Senate House Library has strong collections in languages, the arts and social and historical sciences. Any registered member of UCL can join the library and borrow from there. You can pre-register online.
The British Library is one of the five legal deposit libraries in the UK, meaning it must hold one copy of all books published in this country. Its collections are immense and impressive. UCL staff and students can apply directly for reference access, although please note that there are currently Covid-19 restrictions. Additionally, the Business and IP Centre manages many business and management subject resources, some of which may be available away from Euston Road for readers.
City Business Library is a public library supporting the City of London. Anyone can use the library in person for reference access only, but there are currently Covid-19 restrictions in place. There are two types of memberships which give different levels of access to electronic resources remotely. A full list of subject specific resources at this library is available.
The Interlending & Document Supply service can be used to request items not held in any of the UCL Libraries. On the Interlending & Document Supply service webpage you can find more information on how to request materials.