I reckon there is a scam by MOT garages fitting pattern cats unnecessarily. On 2.0 Omegas the OE cats cost £880 plus VAT from Vx, and should last 100,000 to 150,000 miles. Yet garages fail them and sell punters pattern cats costing £100 or so, which last about 1 year. Last year I bought two 2.2 low mileage Omegas, confidently expecting to get original cats with them. No chance. They both came with Timken pattern cats. I understand that original cats have a good scrap value (full of platinum gauze) but that pattern cats have no scrap value.
In an MOT it is important that the tester gets the cat good and hot before final test. I suspect some do not bother. I always watch my cars being tested. My daughter took a 2.0 for test in Enfield, London, and it failed on emissions, new cat recommended. I told her to bring it back to Somerset, take back a different car, and I took her car to my local MOT shop. After a warm up, it passed.