I've ridden many sportives at this distance in the UK and here is my experience.
Looking at the route map there are 3 food stops and 11 water stops so you don't need to carry loads with you. I'd say a 750ml bottle will do. If you find you drink a lot on your training rides then take a second bottle.
Food will be available at the stops but you should take some food with you. Find out what's going to be on offer (its usually bananas, jaffa cakes and cereal bars) and bring something different. I tend to buy a tub/box of bite-size flapjack pieces from the supermarket and stuff about a dozen or so in a jersey pocket (can have unpleasant effects if its really wet).
A basic multi tool for adjustments while riding is useful.
Two inner tubes and a set of tyre levers. Small pump. If you get a puncture then just swap the tubes and pump with enough air to get you to the next service spot, where there will be a floor pump for you to attain proper pressure. I never bother with a puncture repair kit.
Unless you are confident about the weather I'd take a small waterproof. It needs to be one of the small, lightweight ones so it can fit in a jersey pocket.
- keys (if you have a huge keyring with lots of keys, take off the ones you need)
- mobile phone
- a £20 note
- I generally don't take a wallet
- I definitely wouldn't be carrying my passport
- small tube of sun cream
- tools, keys and spares go in a little bag under the saddle, like this; other stuff in jersey pockets
- Use at least one or two food stops or you'll run out of energy
- if its sunny, reapply sun cream when you stop
- have fun, I'm envious as I didn't get through the ballot