I haven’t been to London Zoo yet, but I have been a member for a couple of year! 🙂
I can’t say it is fully accessible but I would imagine these days that a large proportion is; though if you are not very high (tall) in your wheelchair you may have difficulty viewing. My local zoo is Chester which has great access, they even have a ‘changing places’ toilet but I find that my height in my wheelchair sometime gives me difficulty seeing over the barriers, and the lower viewing windows for children in pushchairs are too low!!! 🙂
Last year I visited zoos in Spain and Germany and found their barriers less restricting.