Have I forgotten anything? Doing a London - Brighton run on Saturday morning, so far in the backpack I will have :

Spare inner tubes,
Jet pump,
Multi tool kit,
Tyre levers,
Gel bars,
Fluids,
Cash &amp; Card incase,

Am I going over the top taking a spare chain? Do I need anything else?

By the way I'm doing this on a modified MTB 
29" × 2.1 are now 700 × 38,
Grips are now more ergonomic with slight bar ends 

So more towards a hybrid but with MTB gearing.
You have said you're planning on taking: Spare inner tubes, Jet pump, Multi tool kit, Tyre levers, Gel bars, Fluids, Cash &amp; Card

So you've covered flat tyres, energy, hydration and money.

Other suggested repair items:  chain tool, quick links, spoke tool, zip ties, strong tape.

Unforeseen circumstances:  mobile telephone/cellphone, emergency contact information, rain jacket, hat/gloves

First aid:  some disinfectant wipes, a tube of antibiotic ointment, 2 or 3 sticking plasters, a gauze pad and a tiny roll of tape.  Tweezers.

Daytime Running Lights - You haven't mentioned time of day or season, so consider DRLs for daytime riding and if there's a risk of being caught by sunset. (Personally I use DRLs all the time, and keep some other lights for night time.)

Lock - Is there a chance you'll leave your bike unattended? A small bike lock might help if you choose to duck into a shop or cafe.

Even if you cover up or apply before you leave, a small tube of sunscreen will allow you to reapply on your face, ears, backs of your hands or any other exposed skin.  A number of people I ride with also use a SPF chapstick.