It's been a while since I used the photographic library at IWM, so things may have changed a bit. Anyway, the last time I went, the official photos from WW1 & 2 are in an annexe around the corner from the main London museum. It's there that you go when you visit; so not a question of them having to move files for you to the reading room at the main museum. In fact, you pick them off the shelf yourself.
I don't know the procedure for non-official photos or photos from other periods.
If you've found an image in a book that you would like to have a decent copy of, you could always send the IWM staff a photocopy - they have a superb index system (albeit not by vehicle type!), which helps them narrow down where to look. If you have the reference number, they (or you if you visit) should be able to go straight to it.
The reference number system takes some getting used to. The KID series is the one showing portrait photos of vehicles, essentially 'works' photos. These are in albums according to weight class. Other than that (unless some more work has been done since I was last there) you have to go through each series in turn, e.g. from memory, the H series is the WWII home front series going all the way up to D-Day. There are several million official photos to look at. I spent a week there one year and just managed to get through all the WW2 albums, but very rapidly! I wish I was retired and had more time to do them justice.
Check out the prices of reproduction before you go. You may be in for a shock!