O2 sensors are usually upstream closer to the engine so if it's the back CAT, it likely won't have anything attached. You could put a straight pipe or clean our the CAT substrate and reinstall the shell (if it's even possible to get out the substrate. HOWEVER, I'd suggest getting a new one -- possibly aftermarket or you might be able to find a used one from a scrapped vehicle as well. I'm not sure what they cost in the UK but you should be able to get one somewhere in $300 US Range.

See if Converterwarehouse.com might carry one or discountconverter.com or use a universal application like this:
Magnaflow 99105HM Universal Catalytic Converter (Non CARB compliant) | Frugal Mechanic

Good luck.