Wrong - its not illegal to modify your so long as it's roadworthy - cops won't screw you over for an air filter or even a remap. The only people who care about mods are the insuance company.
But as mentioned, will they check? I had a crash in Nov, my fault - car went into the garage with LCR alloys, mod'd airbox, revo, full MY03 kit on a 2001 car, LCR interior, Amps and subs and sony HU, boost guage, new dials etc etc, it was clear to anyone with any car knowledge it was not standard. The only "invisible" mod was the Remap, but my SPS3 was probably in the glovebox. Anyway, Tesco's investigator came out and looked at the car and approved the repair.
Not only that but when we had to order the new parts for it, I ordered an LCR bumper instead of the MY03 one (which even that was an undeclared mod in itself). I just had to pay the difference in price between the MY03 and LCR front (about £40 quid or so).
I've had similar experiences in a 206 in the past aswell.
