wouldn't recommend it (obviously... I the mod). Basically, your car needs to be roadworthy (and comply with regulations) to pass its MOT without which you're not technically insured.
However I believe that you'd only not be insured if your car was tested for emissions by the police and they told you to have it fixed. Then your MOT would be invalidated and you'd only be allowed to drive to the repairing workshop and to the MOT test centre.
This is not solid legal advice, and I strongly recommend you to get a new cat, but so long as you don't get cought you should be OK, but others will tell you that its a engine modification, in which case you should be telling your insurer, but you cant because the mod is illegal...
The words 'can of worms' springs to mind. you can always get a second hand one.. breakers usually have a stock of things like that, although there is no guarentee that it will actually work.
EDIT: Only cars built after sept 92 (I think actually need them) so if you've got a J/K plate you may be able to get away without one and not have to tell your insurance company. mmm tinned worms.
