The IWM in London has a super reading room open to the public, and I've always found them very co-operative. I'd e mail them in advance and let them know what you are after, then the files you want can be got ready. Bovington tank musuem have a huge collection of photos, might be worth contacting them as well. The National Army Musuem in Chelsea is a short journey away, so there may be another scource. If your spending a couple of days at IWM, they also have a great cafe, and for London, sensible prices.