You can fit:
- lighter tyres like schwalbe one which may save you 100g per tyre
- lighter inner tube like tubolito (~35g vs std ~80/90g) but when you get a puncture, they are a nightmare to fix (long curing time so you better carry spare tube--> against the objective)
- remove the 4 wheel rack, keep the single wheel mud guards (or remove them if it's dry)
- cables, look how much cable you actually need; cable length are usually too long and could be made shorter but, you need to check that the reduced length doesn't interfere with the mechanism and that the cable does not pinch. Replace the OEM cable with something like Jagwire elite sealed or elite link. Between the shorter length and the lighter material, there is a bit of weight saving.
- you could fit a lighter seatpost (like litepro) but watch of the weight limit, I would not got there if the load in greater than 80/90kg.
- you could fit a carbon handle bar and light grips
- swap the roller wheel for lighter ones
- fit lighter pedal (lightest pedal may not fold)

then, the more complex:
- remove the crankset and bottom bracket, bit a ultegra BB and carbon crankset for a single chainring or a GXP BB with a Force 1 or Red carbon crankset and a narrow wide chainring (I got a 50T with my force 1 by default) but this is expensive (£300)

then, there is a myriad of Titanium bits and carbon bits with various quality (see potential failure, bad mechanism etc.) that will reduce the weight but they are expensive and can spoil the ride.


But As somebody else said it s expensive and you may end up with something that is more expensive that the OEM upgraded version. Also, look at the cycle2work scheme condition; in my case, I was suppose to keep in original (other than maintenance condition) until a paid it fully. All the upgrade were quickly reversible, spare wheel, seat post were fine but I swapped the groupset / crankset when the repayments were completed.

Also, you may want to review the chapter3 spec and use it as inspiration.