Work on the basic principle that NOTHING is EVER like it is portrayed in the media, and you won't go far wrong!
The WI is about cakes and jam and flower arranging, but that is only a very small part of what it does.
Have a look in your local paper - most WIs publish a report of what they have done at their meetings in the local press.
Also, check out the WI website and see where local meetings are, and then ring whoever organises it and ask if you can go along to a meeting to see if you like it.
(I am not a member, but I know people who are.)