Some ideas, at the lower end of the budget;
Conti super sonic tubes -50grams x 2 = 100 grms c. £20
Michelin pro4 comp ltd 160 grms each, poss saving of 150-200grms c. £65
Campag Zonda wheel set 1,550 grams, poss. 500 grms saving c. £270
Fizik R1 arione saddle 145 grms, poss. 150 grm saving c.£200
Look keo Ti blade pedals c. 180 grms, saving 80 grams c. £150
That adds up to around 1,030 grms of weight shaved off for a mere £700
You could possibly shave a chunk more off with a different gruppo but that is going to be more expensive that all of the above combined.
Equally you could get lighter wheels but for a lot more money, but I wanted to try to put forwards some lower cost options.