Brunel Library is a member of the M25 Consortium of Academic Libraries, which currently has 57 member institutions, with a combined user base of over one million people! As a user of Brunel Library, you are likely to be entitled to use many of these libraries as well, either for reference or borrowing (possibly with a small charge).
The M25 has recently launched its new search service, search25, which allows you to search across all members libraries to find items you require. The “user options” page allows you to limit by specific libraries (e.g. just those you can easily get to), and by access type (reference or borrowing, depending on what type of user you are).
If Brunel Library don’t have the item you need in stock, you can contact your Subject Liaison Librarian to recommend it, but you can also take advantage of Search25 to see if it is held anywhere else nearby.