Pros
The British Library is a world-renowned institution with a vast collection of resources that you'll have access to, including historical documents, manuscripts, and other materials that are invaluable to researchers in a wide range of fields. Additionally, you'll have the opportunity to work alongside experts in different areas, collaborate on exciting projects, and develop software solutions that will help advance research and knowledge. The British Library also offers a nice pension package. There is a "nice" and friendly work environment.
Cons
The British Library does not offer competitive salaries, nor does the organisation really care much about career progression or keeping talent in. That stymies professional growth and development, both for the individual and, more problematically perhaps, for the organisation itself.