I agree with StealthPolarBear - it really does depend on your circumstances.
I started mat leave two weeks before EDD which was perfect for me - and is what I recommend to friends in similar situations.
I didn't want to start too early as I wanted to maximise my time after the baby was born and I found it was only when I got to about two weeks before that I was starting to struggle with waddling in etc.
That two weeks before EDD was such a wonderful time - I remember it so fondly! Lots of movies and beauty treatments and sitting around in coffee shops/parks reading books etc. Also used the time to get all the baby's things ready which was lovely. Really relaxed and spoilt myself.
Remember though that you will be pretty big and probably quite tired. I had all these grand plans of trekking into London to see exhibitions and museums but they were all shelved as there was no way I was up to that kind of exertion.
Also in the end DS was two weeks late (relatively common for first babies) and so ultimately my mat leave started 4 weeks before birth date. The second two weeks I remember less fondly as I was pretty knackered and fed up by then!
I would have personally found taking 4-6 weeks before EDD a bit boring and think 2 weeks was perfect.
Having said that next time round I think I may stop a bit earlier so that I can enjoy being with DS on his own for a bit before no. 2 comes around but I will see how I go.