My biggest input to this thread is this: 829 posts on this forum and you don't know the differences between a 172 and a 182? Wow... seriously what have you been reading/writing threads/posts about all this time? I'm sure the below has been mentioned already...
PH1 172
Cable throttle body (a purer driving experience)
Lighter (as above)
PH1 bumpers, lights, etc etc
OZ F1 alloys
195/50/16 tyres (tad cheaper & a bit more choice)
"RS" logo stitching in seats is yellow/blue/red instead of grey in the PH2/182
Insurance group 16D (i.e. 17)
PH2 172
1.2 Look-a-like Wheels
195/50/16 tyres (tad cheaper & a bit more choice)
Stealth exhaust (Only circa £200 when it falls off)
4-1 exhaust manifold (Should you ever need to, it takes ages to remove)
Insurance group 16
182
Wheels (IMO more interesting)
205/45/16 tyres (tad wider thus more expensive)
Twin exhaust (IMO looks better. Costs circa £400 when it falls off)
4-2-1 exhaust manifold (Partly responsible for an extra 10 HP. Should you ever need to remove it, you actually stand a chance)
ESP, Traction, Cruise as standard (172 got ESP in 02 & cruise control in 04 IIRC)
Insurance group 16 (However because these are newer, significantly higher value and less are in the hands of crashing pikeys these DO seem to get lower quotes than a 172. It's not all about insurance group...)
If you want coilovers, forget the cup-packed or cup 182s. If you want luxeries get a 172/182. If you want pure-ness then a PH1, 172 cup or 182 trophy. If you don't want poverty spec then as said everywhere the Full Fat 182 is "the" one to go for.