Ask anyone who has stripped there car to the guts and they will all tell you, Its the best performance modification you could do short of dropping a Bigger motor in. Ive found that its not just the interiour you can strip, If you have big bumpers, Try getting under the motor, There will be a small metal plate at the front, Thats basically the front of the car on small bumper models, You can take this out and replace it with just a bar to keep it riged, You will find the same at the back too, The front wheels have mud catchers round them, These can be removed in seconds if your prepaired to cut them out. Also, Rims are an important way of lightning a car, OZ are the best performance rims if your on a budget, If your gonna be driving hard, Get the turismo or Superlegger multi spokes, As there gonna give you the supporting strenght.
Also, Invest in a 3 piece strut brace kit to make your car more ridged, Im waiting for mine to arive so ill give you some feedback on how they go.
Another thing i can recomend to you is the peddles, People say the halfords peddles are cheap boy racer tack, But ill never drive my car fast without them, There a real saver and reduce the travel time of your feet. A real must in my oppinion.
Carbon fiber should also become your new best friend
But if you cant afford it, Just take off your bonnit, takes 5 mins and reduces the waight a lot. Also, behind the grill you should find a small patch of plasic blocking off the part of the grill not covered by the radiator (if its a stock grill) Removing this works wonders for getting air to your Cone Filter if you have one.