I guess my dreams of moving to Austraila are over.No...not actually 'declare'.
But...if you're pulled over for a 'licence check' then sphincter factor kicks in, and there is a huge vacuum seal between your arse cheeks and the leather of the your seat while Constable Care inspects your vehicle, does a random breath test (alcohol/blood %). If he even thinks that something is not right, then he will slap a yellow sticker on your windscreen and you have 14 days to present the car with the noted 'fault' rectified, or you take the car to the Licensing Centre and plead your case. In past experience, it's best to fix the fault.
Also, an average eagle eyed insurance accident assessor will know what is what, and will deem the modification non-compliant for insurance and bang. No claim, and you're paying for the damages of any not at fault party invovled in the accident/claim.
Simply...not worth the hassle here.