The Institute of Historical Research was founded in 1921 and is one of eight institutes that comprise the University of London’s School of Advanced Study. The IHR is dedicated to training the next generation of historians, and to producing and facilitating ambitious, innovative historical research.
https://www.history.ac.uk/
This was quite a new source to me, and while its contents are mainly for students of the institute and only available onsite. there are quite a few links to potentially useful websites. I would recommend starting with the Online Resources page. Well worth bookmarking!:
https://www.history.ac.uk/library-digital/collections/online-resources