I'm going to split up my photographs, to cover the different exhibitions in depth, but I just wanted to briefly give an overview if you are considering visiting IWM.
The museum is free to visit, open daily 10:00-6:00. Due to the high volume of visitor at the moment, it is timed entry for the WW1 exhibition, but again this free (but still very crowded).
Photograph is allowed in the museum, apart from (and rightly so) in the Holocaust exhibition. Another thing to note- the Holocaust exhibition is not recommended for children under the age of 14, and includes graphic photographs.
We spent about 4 and half hours in IWM and didn't manage to see all of the exhibitions. We didn't go in Truth and Memory or A Family in Wartime, simply because we ran out of time.
I'd recommend you bring your own food and drink. There was a poor choice of sandwiches and both the tea and coffee are horrible.