Get a 100mm long stick/bit of wood or metal (or coke can) and move it around under the car. If this does not fit (including under cannons/cats) then it is not legal. Making it 100mm or higher will not prevent you from getting a yellow as the cops will say "It
looks too low". It will only help you pass pits.
I am currently poor spec so I don't have any engi certs for my coilovers although
afaik the law has changed and now coilovers are illegal without a permit (before you could wind them up to legal height and soften the damper).
I got them from Evolution R for ~$1350. There was a 17cm gap in the wheel arches beforehand and now there is virtually none.
I have been pulled over a few times and given death stares by plenty of cops but have not been stickered yet. In fact, I got pulled over more when it was stock height, however that was probably due to the fact that I used to drive like I was in fast and the furious.
I'll upload a couple of pics if you want.