Here is one small hint re: the Maps Reading Room in the British Library. The types of available maps are quite mind-boggling, and you will want to spend the rest of your life in this room, just looking at maps. However, eventually you will be forced to leave, and you will probably wish to take copies of some maps with you. Unfortunately, photocopying/photo-reproducing maps at the BL is incredibly expensive. Even if you have a very generous grant, you will feel very poor the instant you place an order for a photo reproduction. And, even if you could afford it, you will not be allowed to copy any of the maps from the King's collection (Shelfmark Maps K.Top.115.5., for instance). So, what I found was that the Maps Reading Room was great for looking, but not so great for buying.
Although it is not ideal, you can often find scans of these same maps on the web. Some are printable, some are not, but here are a couple links to get you started.
- India Maps in the David Rumsey Map Collection.
- Perry-Castañeda Library Map Collection.
- Digital South Asia Library, Imperial Gazetteer of India.
- Roots Web Historical Maps of India.
- South Asia Maps.
Also, don't forget to check out the catalogue for special collections and maps at your home institution. While there are indeed a lot of maps in the British Library, it is likely that at least some of them are also available at home. Don't waste precious time looking at things you can see before/after your research trip!