There are some subtle, simple and cheap mods that can make a big cumulative difference cosmetically - smoked or clear side indicators, de-badging the back end, removing dealership stickers, plain tax disk holder, non-branded number plates and so on cleans things up a lot, Ice blue headlamp bulbs and sidelights, chromed indicator bulbs and so on can make a subtle difference depending on the colouring. A heavy tint on the rear windows and boot helps blend the 5-doors' two rear window panes together and makes the car look a bit longer, flatter, wider and sleeker (couple of hundred quid for a pro job or about £30-40 if you DIY) and a medium-dark spray tint on the rear light clusters polished-up makes the lines round the car's backside look smoother. Rally mud flaps (my next mod) can make it look lower and wider.
A good wash, t-cut, wax and polish can do wonders too.
If it's a phase 1 (early 2003, no plastic grille on front spoiler, separate front headlight and indicator housings) you can use no-more-nails or something to put a mesh grille on the lower bumper quite easily and if you have the foglight blanking plates you can slice off the back of them and put mesh behind them for a nice effect (or a neat place for cold air feed if you use a cold-air kit and route the ducting where the resonater lives). Even something as simple as changing the interior lights for bright white LEDs can make the inside a nicer place to be.
You can get a decent set of lowering springs for under £200 which gives a subtle drop and improved handling (though you need to find someone cheap to fit them, I got raped on that).
You can pick up a nice backbox for under £200 but THB if the exhaust is in good shape a well camoflaged exhaust tip is a cheap cosmetic mod that makes a difference.
Add a cheap set of 16" alloys (lots of good deals around right now) and nice tyres and you should have change from £1000.