Its not illegal to have no cats as long as the car meets the emission level required to pass its mot, not just on the mot day but all year round..
My insurance company told me they would insure me with a full decat because its not illegal. However they said if i made a claim the wouldnt pay out because the car "probably" wouldnt pass an emissions test and as a result the car wouldnt be roadyworthy, therefore the MOT would become invalid, and without an MOT the insurance would become invalid.
Its one way they can insure you but refuse to make a payout.
Al