The NOx fix has nothing to do with the MoT. But I would be very wary of submitting a car that had done 123,000 miles to a 'fix' approved for used cars in very good condition with modest mileages. What tends to happen is that the worn injectors cannot cope with the more complex injection sequence, the ageing EGR cannot handle the increased soot and the DPF then has to regenerate more frequently to burn off the soot and more rapidly fills up with ash. At 123,000 miles the DPF must be near its capacity for ash anyway.