taken from 7mat's post on renaultsport forum and very well put imo:
It is only a legal requirement for manufacturers to release cars with cats fitted.
Removing your cat is not illegal, and is not a valid reason for an insurance company to walk away from a claim. Although not reporting your mod’s to your insurance company / broker is!
Its true insurance companies do no like to insure cars without cats, but not because it’s illegal just their policy.
Checking to see if a cat is fitted is also not part of the MOT process.
If your car can pass the emission test without a cat then its road legal and insurable.
It’s a bit of a grey area but until the laws are changed regarding the use of cats then we can all get away with removing them.
All information from the UK MOT Test Magazine website;
6. Your vehicle has had its catalytic converter removed and replaced by a normal exhaust silencer but passes the emissions Test. Does it fail the MOT because it hasn't got a catalytic converter when it should have one?
a) No, if it passes the emissions part of the MOT that is acceptable.
So if your car can be MOT legal without a CAT, then it stands to reason that it can't be an insurance failure.
http://www.motester.co.uk
http://www.motester.co.uk/motquizans.html
My car is fully insured including mods as the policy states car fitted with a full aftermarket stainless steel exhaust system.