I have identified some archives and organisations collecting photographs that can be interesting to explore in the future for work ideas:
The V&A Prints & Drawings Study Rooms (South Kensington, London): This houses an important collection of 19th and 20th century photographs amassed by the museum over decades (the V&A has been actively collecting photographs for over 100 years), which was recently enlarged by the transfer of the RPS collection from the Science Museum. The original photographs, including those made directly on glass plates, can be accessed by the public under certain circumstances and are grouped in boxes, with some of them contain work by unknown photographers, so this could be an interesting way of discovering new artists and ideas that could perhaps depart from the mainstream.
The National Archives (Kew, London): The archives have various materials and documents as well as photographs that could be accessed by the general public for viewing. Some of the photographs can be searched online, but viewing / downloading incurs a cost, so it may be better to view them directly in the archives’ reading rooms. The selection of photographs do not seems as extensive as in the V&A, but the combination of photographs and documents could provide interesting ideas to explore.
The British Library (St Pancras, London): The library holds an important collection of photographs, including the archives of Fox Talbot, one of the medium pioneers. Like the V&A, this is another good way of getting to know early era photographers, but obtaining a reader ticket seems to be more complicated than in the case of the V&A, so I would probably leave this as an option for later.