I think it's all down to 'taste'. 'Chavving up' entails going totally overboard and making a car look ridiculous. The mods also have to be done badly, so the end result is a car that looks like cheap hatchback trying to be a race car covered randomly in parts that don't compliment each other.
Tastefully done mods can look good and enhance a cars appearance. A decent body kit that enhances the lines can also look good. Frosty was lucky to find a bodykit that actually makes the Uno look good rather than fit front and rear bumpers that make a Uno look like a snow plough!
And Chris has made his Uno look nice too by making subtle changes and improving the lines of his car. There are also other tastefully modded Uno's on the board too, sorry if I forget who they all belong to.
Also free-flow rear exhaust back boxes are fine if a tasteful one is chosen. You see some cars with a back box the size of a drainpipe hanging under the car. It looks all out of proportion! Choose a bigger one by all means, but don't go overboard with the tailpipe size.
I also agree with Alex, front fog lights don't actually serve much purpose if used correctly and within the law. Check the highway code and the vehicle constructions and use act to see the conditions when front fogs are legally allowed to be used. You'll see that for about 95% of the time driving conditions don't actually require the use of front fog lights, and if used when not needed it is actually a road traffic offense that can get you stopped by the police and fined.
However, 'Chav's' seem to use front fog lights as a means of identifying themselves to other Chavs. If you see a car in broad daylight or clear night conditions with front fog lights on it is almost certainly driven by a Chav!
Oh, and Alex, it makes sense to keep the front plastic wheelarch liners on the car in the UK. Without them, the inside of the front and inner wings get covered in muck, and before long they start rusting away. With our grotty/ damp/ muddy roads we need them to protect our Unos!
At the end of the day it's all down to an owners individual taste. I wouldn't be seen dead in a Chavved up motor, but a well modified car can look great and draw respect and admiring glances from others.