Oh hey I'm going to England as well! Then I'm going up to visit my family in Scotland~
Maybe you could get some near Buckingham Palace? You might be able to get some neat ones in Hyde Park as well. Though if you are planning on taking pictures of cosplays with the Eiffel Tower, I wouldn't recommend trying to actually be on the tower while taking pictures, especially since it's always extremely crowded to get to the top and when you do get there, people usually will want you to hurry up, and it would probably be impossible to get any without a bunch tourists around you in the photo. I think ground level shots of the Eiffel Tower would be best, personally. Much less of a headache.