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 a strong collection in bibliography, especially historical bibliography and reference materials. The Palaeography and Manuscript Studies Collection contains one of this country's richest collections for manuscript and archive studies. You can pre-register online.
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.