[Mole]

Yes, it is a can of worms and there's no simple answers.

Everything used on a UK public road needs to satisfy the requirements of the Construction and Use Regulations and the Road Vehicle Lighting Regulations at ALL TIMES. An SVA test consists of some (very) basic visual assessments of whether the car complies with these. If you alter it after the test, it might not comply. If it doesn't, it will be illegal. The police are the only chaps who can prosecute for breaches of C&U and mostly they stick to bald tyres, noisy exhausts etc. so it is highly unlikely that anyone will get caught. If, (in my (very limited) experience of insurance companies), there's a big accident, they'll clutch at any excuse to avoid a big payout.

There is one important issue, however, which muddies the waters somewhat.

Construction and Use contains almost nothing relating specifically to interior or exterior projections - there's a bit about mascots but that's it. As a result, centre-lock wheels with big spinners, big door / bonnet handles, filler caps etc. should all be OK. The only bit of C&U they might be able to pin on you is Regulation 100 which is a catch-all that forbids anything on or in the vehicle to pose a danger to occupants or other road users. That being the case, it would end up being for a court to decide whether or not your particular modification did or didn't constitute such a breach. I strongly suspect that (in the worst case) if a medical expert witness told a coroner that Mr. X died -even partly as a result of lacerations sustained after impact with a particular sharp bit of your car, they would "throw away the key".

Don't know if all this helps (or not) but I do feel it's a problem -not just relevant to Cobras, which really does need to be aired...